My imac g5 is constantly going to sleep because of overheating. I can really only use it for about half and hour or an hour max before it just shuts down. I recently (in the last two months) had to have the HD replaced, the logic board and the power supply because it kept freezing. Now it's been doing this overheating thing for weeks and I'm extremely frustrated. I ran techtools but nothing came up. Does anyone have any ideas? I've been monitoring the heat and fans with istat pro and the fans are up to 5600 in no time and the temp is consistently rising. I hate to go back to the apple store (I have apple care) but last time it took me three "fixes" before my machine would stop freezing (1st: HD was replaced, 2nd: They said nothing was wrong and sent it home, where it promptly froze, 3rd: Replaced logic board and power supply).....and now it goes to sleep because of overheating. Ugh . Because of my schedule it's hard to make it down there and don't want to make the trip unless absolutely necessary. Does anyone have any ideas of what the problem could be?? Thanks, allison.