I upgraded my 2008 iMac to Snow Leopard not too long ago, mainly so I can use iLife 11. However, it seems that my iMac is starting to become unstable. The occurrence is random, sometimes even while I'm doing nothing, but the iMac will lock up (frozen entirely, mouse not movable, sometimes with a spinning beach ball seconds before, sometimes the iMac's screen just turns black) and my secondary display will show a green screen with some vertical lines (this happens all the time when the iMac locks up). Any ideas? This is getting annoying as I never had this issue on Leopard. I ran the Techtool diagnostic, and everything is shown to be fine.