maybe win 98 disagrees with your system for some unknown reason, i had a similar problem with win 95 and an old cyrix that i now run 98SE and redhat on, if you are using the original '98 maybe you should try SE.

could also be your onboard hardware causing the trouble, although with those specs it seems unlikely, but its still a remote possibility, the comp that i was having trouble with strange as it seems has an unbuilt C-media chip too, maybe it could be an issue with the sound chip, but that still doesnt explain the unexpected shutdowns though

are you sure that the PC isn't running too hot? some new stuff shuts down automatically if it gets too hot and i know that AMD's run a bit hotter than others and they have the facility to detect the temp of teh CPu i think, that may be your problem, but these are just some suggestions

hope they are of some help