For a while, I had a problem where the LCD monitor on my main computer would suddenly start flashing from whatever it was displaying at the time to black and back about twice a second. The weird thing was that the machine itself appeared to be "asleep" (i.e. not doing anything) while the screen was black. The problem appeared to be limited to Windows.
Thinking it was a problem limited to Windows -- like a driver problem or something -- I decided to spend quite a bit of time in Linux for a while to confirm my suspicions. As it turns out, there was a different problem -- the screen went black as if it had been put to sleep to save power, only there was no waking it. In fact, I had to do the magical Alt+PrintScreen REISUB reboot. Upon reboot, the boot text was garbled.
I couldn't find anything anywhere on the WWW about it either problem. I figured the card was bad, so eBay to the rescue with a Geforce FX5700 256MB to replace my 5500 for $35. Not bad. I'm still waiting on the card, though, so I was going to grab an old temporary card (ATI All-In-Window Radeon 32MB) to stand in until it arrived. When I pulled the 5500 out of the case, lo and behold, an entire family of dust bunnies had taken up residence in the cooling fan of the card! I figured this might have been the problem, so it got a careful once-over with compressed air, a quick replacement back into the machine, and... everything seems to have gone back to abnormal.
Moral of this story: dust bunnies aren't just cute, they're electrically delicious.