W98se reboots continuously

I run a number of NEC PCs (600Mhz 128Mb RAM). One of them has begun to reboot continuously – so it would start normally, load Windows and show the desktop as usual, then after a few seconds the screen clears and the whole process starts again, and again, etc. Can boot in SAFE mode and have run disk check and defrag.
Suspected a virus or spyware and ran Sophos in SAFE mode – nothing.
Eventually reinstalled Windows. Now it still does the same except it restarts when the first ‘New hardware found’ is reached.
Tried substituting components from another NEC machine but the fault is always present on the original one. BIOS settings are also identical.
In summary the PC with only W98 installed (can’t load drivers or apps)continually reboots.
Help – any ideas?