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My suggestions would be first, RAM, second Power Supply. This of course is just based on personal experience and pure guessing based on what you've given us so far. It could theoretically be anything, including an IRQ conflict.

sounds like something is overheating or the motherboard sucks.. try to buy some arctic silver thermal paste, look up a guide to applying it if you're unfamiliar with the process.. it's pretty simple.

also try to improve the computer's airflow; keep your wiring job neat and out of the way if possible, have an exhaust fan plus an intake fan to keep air flowing. make sure there's no dust in the cpu's heatsink and the chipset's heatsink - can be a significant source of heat if dust clogs the hsf up because it prevents airflow from cooling it.

It might be your memory.

Some boards have problems when the memory and FSB are out of sync.

You might try underclocking the memory to 333 to match the FSB. I remember reading somewhere that synching 400MHz memory down to 333 can actually improve overall performance with a 333 FSB AMD system.

You know what I had a computer do the exact same thing, and this is how I fixed it.

First off I would like to start off, that Biostar sucks, get a new motherboard. (not all suck but 95% do suck) I would suggest maybe an Abit or Soyo or Shuttle board, something basic nothing to extravagant.

Next if you don't want to splurge on a new mobo, try moving the ram around on the slots, maybe 1 slot is just messed up, I've seen that happen on two or three systems before, and just by moving the ram one slot over fixes the problem.

If you are trully overheating, then your system would shutdown, or better yet melt...But Airflow would be addition to a case, and definetly helps in the long run.

Is your copy of Windows XP Legit, The reason why I ask, some people don't like to download updates for that reason. Although you can download updates. But It also depends on what version of XP you are running. (I don't mean to assume you are running XP).

Now this is how I fixed my problem: I first re-installed windows with a clean version of Windows. Perhaps to Corp edition on XP pro. No added whistles...nothing...then in this order I installed my software and updates.

1.Directx 9.0c
2.Sound Drivers
3.lan drivers
4.video card drivers. (now you probably don't want to use Omega drivers, But I like them a lot, But I recommend using ATI specific drivers for this installation)
5. Windows update to SP1
6 Windows Critical Updates
7. USB drivers
8.Windows SP2 drivers
9. AVG
10. Outpost(agnitum firewall)
11. AD-Aware 6 pro
12. Window Washer

Now let the system run stable with nothing else installed on it, just the basics for 1 week.

Next install a simple game to test, perhaps Star Craft or Operation Genius and run it...let it stay running all day. Also install any updates necessary to the games.

Now after running for about 2 or 3 weeks, you should be set to go, If not, then try replacing the mobo. and use good thermal paste for heat dispertion