My iMac DV 400 has also somekinda monitor problem. If you are runnin' some heavy applications (Adobe GoLive, Photoshop or sometimes Toast) and you let the computer go to sleep, it won't wake up. You have to press many keys and wait about 5 to 10 minutes and then it might wake up. Depends if the machine has a good day.. Then it will work.
I fixed this problem: I don't let the iMac go to sleep, but I'd like to know that why does it do this. Why?!