ok, here is what i now recommend. First, set that voltage jumper back to the default. now go into the bios and use the "set all defaults" option. Reinstall windows 98, and do not install powerstrip. Install the 4in1 drivers, then install the latest detonator drivers. What version of the detonators are you using now? Also, check that your video card is seated properly. I would remove it, and reseat it.
Are you running other software in the background? What type of cpu cooler are you using? What brand is your powersupply? The powersupply could still easily be the porblem, no matter what the salesguy said.
You might also try updating your bios to the latest version.
If the problem persists, check the ram clock speed and the CAS setting. Is your ram rated at CAS 3 or 2? Try running the ram at 100Mhz CAS3 for a while, just to see if that fixes the problem. If none of that fixes the problem, try installing powerstrip again and try downclocking your video card.
If none of that helps, then I would recommend getting your motherboard replaced. If there is still a problem, then mabe your video card needs to be replaced.