That's a sign of a well known issue with XP and DMA. I've written about it many times, and no longer write the long answer. All apologies but PLEASE DO THIS!
If you want to do more research, please do that.
I have Win XP with a video card I installed (unk specifics, I'll add them when I get home later). It also is functioning as the soundcard. Lately when I run any online video, Windows Media Player, or try to browse multiple websites, my monitor says "No signal detected [from vid card]. Turning off" The computer stays on. When I cold-reboot the computer everything comes on normally. Also, sometimes when I'm playing music the audio cuts out like a CD skipping. Any ideas?