Constant Lock Up’s

We have a AMD K6 233, 32 meg SDRAM, Win 98,
and the machine constantly reboots, and sometimes hangs for no apparent reason. Suspected EDO memory so replaced with SDRAM. SUspected shared video memory so installed a 16 MEG PCI graphics card. What else could it be? The motherboard does not support sleep mode. It will work for hours one day, then the next hang up every 5 minutes. Struggling!