Computer Doesn't POST

I have an Athlon palomino XP 1700+ with a MSI motherboard, KT7266 (VIA
chipsets), Win 98 SE. It has been running relatively problem free for about
1.5 yrs, until recently. It had been getting sluggish, with occasional
lockups, so I reformatted the drive and reloaded win 98 SE.

Most of the time it works okay, but occasionally everything locks up. Then
the only way out is an impact shutdown - power off/back on. Sometimes when
it gets to that state, hitting the reset or power buttons won't do anything;
it has to be unplugged/plugged back in. Then after powering back on, it
starts to POST but hangs where it says "checking NVRAM".

I checked CPU temps, and the max is around 58°C, which I think is where it's
run before under similar room temperatures. I did find that if I shut it
off for about 10 minutes, it posts okay. Seems like a thermal issue, but
why would it have started happening only after I reloaded win 98? Or any
ideas on what it might be other than thermal? Thanks.