This sound like Power Supply or Ram problem Or

Been having some issues with Compaq Deskpro EN PIII 933. 512 MB Ram, XP Home. It's been running fine until a couple of days ago. Originally got an error:

*** Hardware Malfunction Call your hardware vendor for support

NMI: Parity Check Memory Parity Error

*** The system has halted ***

And would thereafter only boot to Safe Mode. Wouldn't do a normal boot at all.

After uninstalling-reinstalling the sound card, video capture card, the computer seemed fine again. However shortly thereafter it started exhibiting a new symptom.

I haven't gotten that blue screen again, but it just winks out in the middle doing something. It's done it three times now - twice when I hit the tab to go into capture mode in Pinnacle Studio, but it happened a little while ago when I was just typing in the name for a file to capture. Monitor goes blank, computer quits whirring. I get only a light on the front, and I think the power supply fan keeps running but otherwise the computer is dead. Unlplug it, replug and restart, it will boot but now I can't trust it. I have no idea if this is in any way related to the problem I had previously with the NMI error, or just coincidence.

This sound like something that could be caused by a hitchy power supply or perhaps the RAM? Or....?

Thanks for all input.