Definitly sounds like a power issue or a faulty agp slot issue... (whoa deja vu! ) Did you try the monitor on your bro's computer? Did it turn off?? if so then that indicates it's your monitor... Is it at all possible that you could get a PSU that you know works and swap it and see if the monitor still goes out? If it still goes out then it's a faulty agp slot...

Or you could use a method of checking the PSU that I just learned lol... Take out all the stuff you don't need... disc drives, floppy drives, NIC, etc, just the bare minimum... RAM, HDD, Vid Card, CPU... and see if the power saved is enough to notice a difference ...

But first try the compressed air, it's amazing how many problems a simple dust bunny can cause...