I don't know what OS us use, how much RAM you have, how many layers you run or the amount of filters, etc... you run on those layers but, your iMac likely has 256MB of RAM on the GPU. And, like you say it will run the software but, with the odd glitch or artifact the GPU is the likely culprit, although system RAM can also affect the video performance. I had a similar issue on my 2006 Mac Pro using Photoshop CS 5.5 also running 256MB GPU but, only 2GB of system RAM, I upgraded my system RAM to 6GB and really haven't had any issues since. Hope this gives you some insight as to what to start looking at on your Mac.