Ok, i have had a similar problem, on a computer that i've spent the last month or so fixing, you've said what tests you've done, but not what hardware you have, do you have a dual core or AMD processor? i've heard of the same problems with these processors. A test was done to compare the perfomance of a 4 ghz dual core, to a 3 ghz duo (basically a laptop processor in a desktop PC) the duo was performing about 25% better than the dual core, the reason. Heat. it is a knowen fact that the cooler your processor is, the better the performance will be. it is also a knowen fact that 90% of those random restarts are due to overheating, or overworking the processor. in some motherboards the temperature is misreported in the BIOS, so unless you are monitering heat with a digital thermometer, id try changing your fan or getting an aztec case (or similar) with a cooling system. in the mean time, get a pedastal fan and take your case cover off, use the pedastal fan blowing in the case and see if you still have the same problem. also try running a different operating system, you may find that windows is reporting hardware issues where there isnt any, or that your harware isnt in sync with your software. i recommend knoppix (google it) knoppix is a live cd so once you've burned the cd, just stick it in you're computer and it will run linux instead of windows, when you want to go back to windows, take out the cd. (you may need to change bios settings to boot from cd) if your computer still restarts when using knoppix, try the fan idea, while running knoppix, if it fixes the problem, you need a better cooling system, if running knoppix without the extra fan, adn that seems to have fixed it, format your hard drive, and install windows again (get a disk image of the windows 98 startup floppy, the format on that is more reliable, although it formats in file allocation table, rather than NT file system) if reinstalling windows doesnt work, you may find that the problem has nothing to do with bad hardware, or software, but it could be human error, reset bios settings to default, and check your settings in comparison to the other computer you built, also check that your harware is set up exactly the same way. if none of this helps, the only other suggestion i have is reseating your motherboard, make sure that there is not seats (especially if they are metal) shorting out the tracks on the underside of the board. if none of my sugestions help, im of no use, coz i cant think of anything else mate. sorry!