|Could be a failing power supply or faulty ram.|
If you have craigslist in your area, check for people that will diagnose your PC for cheap, most don't even charge for that.
Unless you are willing to try a different power supply and make a memtest CD to test your ram for consistency.
Another thing you might try is removing all the PCI cards and seeing if that helps. If it does, then slowly put them back in...restarting each time till you find the problem.
Some HELP in posting on Computing.net plus free progs and instructions Cheers