Chosen Solution
The iMac switches on and boots without any problems, then after about 15-20 minutes the computer screen goes lots of funny colors and anything the computer is doing stops working. for instance, if the computer is playing music the song will get stuck on the second it switched to the purple green screen. I thought it may be a heating problem because once the computer has been switched off after the problem it does not start again properly. I cleaned out all the dust stuck in the fans and the rest of the computer to no luck. anyone have any ideas?
Try zapping the PRAM and a NVRAM reset Hold (command-Option-P-R) at restart through three chimes. Followed by a (command-O-F) restart hold the keys down until text appears on the screen type “reset-nvram” (no quotes) hit return type “reset-all” (no quotes) hit return see what happens. If neither of those work then it could be the video card is going, OR that the thermal paste needs replacement. causing video problems and freezing due to overheating. Good Luck, N.
