I know this is an old thread but I wanted to chime in here. I have a mid 2011 21.5" iMac with the AMD 6750 M running Sierra 10.12.6 and have been dealing with this issue for over 2 years (upon transitioning from mountain lion to Sierra.) I did have a power supply slowly die in this machine that I replaced with a pull from a working iMac, and that fixed the power issue but the pixelation and glitches started after upgrading to Sierra and seemed to be un-related to the power supply.
This issue has slowly gotten worse over time. I have never been higher than 10.12.6 because my audio software is only compatible with this OS so I'm at an impasse. I have spent the last week trying to diagnose and figure it out. As with the above posters, the computer runs flawlessly in safe mode and the internal Mac hardware tests come back reporting the GPU as "ok." I have tried a clean install of Sierra on a separate partition of the HD but that still produces graphic glitches. 3-4 days ago, the computer had gotten so bad it was unable to even boot into the desktop without locking and crashing.
After removing some larger files from the internal HD and doing some general cleanup, doing some first aid on the partitions I started looking for software issues, particularly things that may have been causing kernel panics. As of today, I've gotten the computer to actually run on my normal Desktop (not safe mode) and that only occurred after disabling some startup processes and/or LaunchDaemons and going through the kexts looking for issues. I've gotten the computer back to a more intermittent graphic issue where I'm typing this on it in Safari at the moment with no issues.
For previous or future posters that may read this, I'm wondering out of curiosity if any of ya'll have (or had) Adobe products on your machines? Specifically the CreativeCloud and any graphics programs (photoshop, indesign, acrobat) because I'm starting to suspect those are at very least contributing to the problem. Disabling most of the adobe background services and removing a couple adobe cache/plist files as seemed to improve things which makes me curious. I have multiple AdobeCRDaemons that run no matter what I do and a kerneltask process is often using up a ton of memory (I have 16GB) without anything Adobe even open (so not sure what that's about.) At very least, Adobe is a resource hog and drives me crazy. I make an attempt to quit all those processes when I start the computer. My next step will to be completely removing ALL adobe software and any remnants to see if that has any impact followed by a total system wipe to factory default, and back to a fresh install of Sierra.
Like many of you my troubleshooting efforts seem to lead me to believe this is some kind of software related conflict, particularly because in Safe mode there is not a SINGLE trace of the pixelation or glitch issue whatsoever as highlighted in this thread. In the past I can also confirm that maximizing an app or safari window would temporarily solve the issue, but then that no longer worked. I also found that when things started to get really bad putting the computer to sleep and then re-waking it would fix the issues temporarily. Additionally, I find that when using Pro Tools (which is an insanely resource heavy audio recording program) with windows on two different screens and a lot of information flying around, the issue was barely present if at all (which is a head scratcher.) Based on information around the web about this AMD card it wouldn't surprise me that the issue is related directly to it and/or it's drivers, but it sure seems like manipulating things in the software realm has an impact on the problem. If I can't figure out a fix I guess it may just be time to put this thing to sleep permanently, but she's had a good run.
