iMac 2010 11,3 Green/Purple Pixels

Hi Guys,

Been browsing the internet for ages, including the apple forums, and always come across similar issues, but never seem to fit mine 100%.

I've got a 27" iMac 2010 11,3 i5.

Basically, about a year or so ago, it started to get these slight green and purple pixels on the screen, more square artifacts than anything else, but it was no problem, because if i'd 'select' them with the cursor, they'd go, as time progressed they've gotten worse and worse, to the point where they would take over the whole screen, again, a simple 'select' all of the screen would remove them for a short while, they never appeared doing anything specfic, but would be guarenteed to turn up.

The system now is only really usuable during Safe Boot, Under normal boot conditions, it will last between 15 seconds and an hour tops, Until it just hangs, the entire GUI is locked, however the Cursor will move, I've left it for a while to see if it will reclaim itself, but to no avail, the only option is to hold down the power until it shuts down.

Now, this only started happening after upgrading to Lion, I've since moved to Mountain Lion, and it still persists, So i decided to do an Apple Hardware Test in Extended mode, I received an error regarding the HDD, which was a 1TB Western Digital Drive, Don't get me wrong, it was slow, but it never seemed to give me issues, so I replaced the hard drive (No Apple care & out of Warranty), with a 2TB Seagate Model, Installed Mountain Lion, instant issues, Straight after succesful install.

I've since done several other Hardware Tests, and the only issue i get is the Hard Drive Sensor, Which I understand because of the 3rd Party hard drive.

So, within the last 3-4 weeks i've reinstalled, Re formatted, Zero'd the hard drive around 15 times, just to see if anything changed.

With Regards to the Safe Mode Boot, Its perfect, never get artifacts, never get a hang, nothing! Same goes for anything other than Normal Boot Conditions.

These pixels can be screenshot and still remain there! They also appear over Screen Sharing.

The system was bought for Graphic Design use, and thats what its needed for, But with it getting to the point of me only being able to work under safe mode, I've run out of options!

From what I know, someting under normal boot conditions is causing the issue, a kext maybe, but I've no idea on which one is causing the issue.

Even if i can get it working with something disabled/removed it's fine, But Photoshop, Illustrator and Indesign are my work, and without being able to use them i'm pretty much limited!

I've obviously tried resetting the SMC/PRAM again, no changes, I've changed the RAM, bought new, swapped out, Nothing no changes.

The system is currently running 10.8.3.

One thing i did notice, is that the system often lasts longer before it hangs under the 64 Bit Kernel, rather than the 32 Bit Kernel (Not sure if thats expected or not but thought it would be worth a mention)

Thanks for all and any advice given in advance!

Ethan.