Unable to boot normally. Will boot to Safe Mode or Recovery Disk.

Yea, once day I had to reboot and it froze partway through. I wasn't doing an update or anything - it's entirely possible I was shutting down and restarting because of a fan issue (once in a while one of my fans will lock at Ludicrous Speed). I haven't tried to regress, but the initial reinstalls I did (from the Recovery mode) were Sierra, and then I since have upgraded to High Sierra, which is what I have when I finally was able to try erasing the hard drive and installing from scratch.

I'll take a look over that article and see if I can find anything. I do remember fiddling with a kext at one point years ago trying to fix my fan issue, but I would think the wipe/reinstall would have cleared that as well?

I thought I had done this before, but whether I hadn't, or if I had and something is different now, deleting all of the .ktext files starting AMD and ATI has allowed the computer to reboot outside of safe mode. On the plus side, I have sound again (finally). However, there is still a lot of graphic artifact from effectively not using the graphics card. (Any sort of video is unwatchable due to constant screen refreshing, Safari since High Sierra has entire sections of web pages blacked out.) I'm going to do a bit more digging and see if perhaps it can use one of the other files be used besides the 4670 and perhaps operate under a lesser feature set, or if replacing with something else would work. If you or anyone else has more information here, I'd certainly appreciate it.

Well, that certainly opened a rabbit hole. While there doesn't seem to be much editable about the 4600 driver aside from disabling ATI PowerPlay and perhaps adjusting the amount of Clock Gating (which I'm not educated enough about to attempt), but I was able to fiddle with a few other things like forcing a rebuild of the driver cache and that possibly this came about because of a change to the drivers with OS-Sierra (as noted by some of the Hackintosh crowd). I'm considering seeing if I can regress just the AMD4600Controller.kext to pre-Sierra (and, if needed, it's dependancies), but given other minor issues, I'm also wondering if I don't just need to get the video card replaced (considering buying a 6970M and upgrading while I'm at it). Part of me just says the bite the bullet and buy one of the newer iMacs (or the $500 Windows laptop that would actually play games decently - the cost disparity is sickening), but this is still a nice little machine...

I don't believe so. Vendor 10de is NVIDIA. The 062e is the GeForce 9600 GT, and the 0655 is the GeForce GT 120. My original thought was that it would have entries for the two cards that would normally come with the device. I'm not sure why it has these two instead. The logic of the link's information is still sound, though - I should be able to affect the GState setting despite the card not beling listed, as I believe this entry is for the computer itself. Going to bed now, may gather the courage to mess with this in the morning.

On a side note - for some reason, I cannot change the brightness of my screen, nor put it to sleep. I previously could in Safe Boot mode. I become more and more intrigued with the possibility of enabling a limited feature set of the GPU...

If I wasn't clear, the layout of my PowerManagement file is different than was in the linked site. In mine, there's an original Key for "IGPU" that has the state registers, while the actual device information later does not:

<key>iMac10,1</key>

<dict>

<key>IGPU</key>

<dict>

<key>BoostPState</key>

<array>

<integer>0</integer>

<integer>1</integer>

<integer>2</integer>

<integer>3</integer>

</array>

<key>BoostTime</key>

<array>

<integer>3</integer>

<integer>3</integer>

<integer>3</integer>

<integer>3</integer>

</array>

<key>Heuristic</key>

<dict>

<key>ID</key>

<integer>0</integer>

<key>IdleInterval</key>

<integer>100</integer>

<key>TargetCount</key>

<integer>5</integer>

<key>Threshold_High</key>

<array>

<integer>80</integer>

<integer>80</integer>

<integer>80</integer>

<integer>100</integer>

</array>

<key>Threshold_Low</key>

<array>

<integer>0</integer>

<integer>90</integer>

<integer>90</integer>

<integer>90</integer>

</array>

</dict>

<key>control-id</key>

<integer>16</integer>

</dict>

<key>LogControl</key>

<integer>0</integer>

<key>Vendor10deDevice062e</key>

<dict>

<key>Heuristic</key>

<dict>

<key>ID</key>

<integer>0</integer>

<key>IdleInterval</key>

<integer>100</integer>

<key>SensorOption</key>

<integer>1</integer>

<key>Threshold_High</key>

<array>

<integer>80</integer>

<integer>80</integer>

<integer>80</integer>

<integer>100</integer>

</array>

<key>Threshold_Low</key>

<array>

<integer>0</integer>

<integer>90</integer>

<integer>90</integer>

<integer>90</integer>

</array>

</dict>

<key>control-id</key>

<integer>17</integer>

</dict>

Props to the PCI ID Repository, btw (http://pci-ids.ucw.cz/). The thing is, AMD's Vendor Code is 1002, and while I can find that in multiple places throughout the PowerManagement file, I cannot find the device code 9490 (or, according to the 4600 .kext, 9488 for the Mobility version, which is what Apple System Profile says I have). All the ones I see are in the 6xxx range covering cards from the 6400M-7690M. Since the OS searches all of the .kext files to find the Device codes for the peripherals in the computer, it makes sense they could be anywhere throughout the file and not specifically ties to one model, but I'm concerned that I can't find it at all, suggesting another is used inherently to run this card.

I did a few searches (I have always loved BBEdit's Multi-File search feature) and found this device not only included in the Legacy .kext but also in the one for the ATI Radeon X2000. I'm really not sure what to make of that. I wonder if I'd be naive to be concerned of a mis-key causing a conflict. There are NO instances of 9488 as a Device in the /Extensions folder at all (since I removed all the AMD*.kext files).

Okay, it took a little trial and error, but I removed the three .kext files that had my Device number. Over several restarts and some trial and error, my computer boots normally, though there is still some graphic artifact, it still runs slower than usual. and games get maybe 1FPS, but it's noticeably faster in general usage than it was previously. The only file I actually had to remove was ATIRadeonX2000.kext. However, from what I've seen this seems to be tied into the OpenCL implementation and FrameBuffering? If true, both of these would explain the remaining issues I'm having. I found some success with similar issues on Hackintosh-related sites, but I'm not sure how, or if, I can translate that practically to my system.

That's just it - only 3 .kext files had the Vendor and Device codes for my card. Two were in groups of IDs (AMDLegacySupport.kext and AMDRadeonX2000.kext), only the 4600Controller.kext had anything about it separately, and it bundled the LegacySupport.kext as well. I've restored all of the .kext files except for the RadeonX2000 and it boots, it just isn't quite up to speed.

I tried several iterations, but if the RadeonX2000 file was in place, it just would not boot, and I can only surmise that the lack of that file (and thus the accompanying GLDriver and VADriver bundles) is what is currently causing my system to graphically be so much slower (and unable to put the display to sleep or adjust the brightness).

I am tempted to look and see if I can find pre-Sierra versions to compare.

Turning off all extensions as you suggested had no effect.

I have removed the third-party user agents, and I generate a new account to test with. Unfortunately, I do not reach the login screen so I cannot access the new account.