I really, really feel your pain. After I posted the above rant I contacted Apple Support last Thursday and went though the usual diagnostics with the support guy who then asked me to try and work in Safe Mode for 24 hours to see if the random reboots occur again. Pointless.
We booted into safe mode and after the horizontal bars effect kicked in I opened Chrome and was presented with a beautiful glitchy screen. Here's a 30 second youtube clip: https://youtu.be/QGt1nGy8yVI
The support guy said this shouldn't be happening and elevated me up to... a higher up guy, who said this was definitely not normal and we should move onto more drastic measures, like wiping, reformatting and reinstalling the OS. This did seem drastic. I told him that I was fundamentally sick of this lemon and that the only process I wanted to go through now was to get either a new machine or a complete refund. He said that he understood my frustration (ha!) and that we had a number of steps to go though before "bringing it to that level". Fair enough.
I was told to get a hard-drive and do a Time Machine backup and we would commence the reinstall. I asked if I could just use one of my existing backups, and was told no, that the backup would have to be reformatted as well to prepare it, etc, etc, etc which I thought was overkill and a waste of time, as I now had to purchase an external and wait for the Time Machine to perform the backup.
The dude said he'd ring back in 24 hours and go through the steps.
24 hours passed, the external was purchased and successfully backed up to and a completely different person rang and I had to bring them up to speed with the situation. We went through the preparation steps, (recovery mode, disk utility, etc) and then he asked me to click erase, which I did, and boom, the erase failed because "Core Logical Volume not found" or something to that effect.
This freaked the high-up technician who said he has never encountered that before, put me on hold for 10 mins and then came back saying that it looks like a hard-drive failure and that I should take it to a repair store and have them look at it "immediately".
Which I did, the next morning, to the same guys who replaced the logic board a few weeks earlier. I explained to them that all I was there for was to have one of the techs confirm that there was a hardware fault and that they were to get in touch with this higher-up guy to establish what was wrong. The kid behind the desk seemed to take all this onboard but somehow none of this information was given to the actual tech who looked at the machine.
Who spent 2 whole days diagnosing that one of the 16GB RAM modules had a fault (and this obviously explained every issue I was having) and because it was third-party materials wasn't covered by my warranty. I explained that I had these issues before the RAM was put in and that all they had to do was try and reinstall the OS and see the error that I had brought the machine in for.
I was told that the installation of software wasn't covered by the warranty and that I would have to pay the charge to do that.
Meanwhile, they finally get in touch with the senior Apple Support "higher up" guy who, incredibly, bizarrely, somehow supplies them with 3 diagnostic reports dated 4 days AFTER I dropped the mac into the repair shop that, inexplicably and unbelievably, corroborates their diagnostics that there is a memory module damaged! I don't know how that is possible.
I drop into the store yesterday with the original Apple 2x4GB Ram modules when all this comes out and, so I am about to go loco on this poor hapless uber-geeky kid when a senior tech comes out of the back of the store, listens to my case and then, like Winston Wolfe, comes up with a plan to solve all the issues.
He's taken the diagnostics on board, is removing the modules, reformatting and testing the mac and will let me know if it is fixed or bust. Hopefully later this afternoon.
If I get an iMac back that is finally fixed (I will not believe it) then fine and dandy. If it is an Apple hardware issue than I can only guess at what the outcome will be.
I am not looking forward to THAT round of negotiations.
Apologies for all the extraneous information. I just need to get it down in writing so I can be clear on the hoops I have had to jump through. I could have done the reformatting all by myself. I kind of didn't want to go down that route but when an Apple Genius tells you to do something...