PC2100 is DDR SDRAM. If you have 2 modules, try removing one (but you may need to be careful about which slot you leave empty!). This will prove if one is bad or not.
Try setting the BIOS to use the manufacturer's defaults - you'll have to nudge the FSB speed up to 133Mhz from 100 though. Adjusting the voltage on an Athlon XP is definitely not recommended, unless you're following one of the expert overclocking guides, such as the one on Anandtech or Sharky's Extreme.
Flashing the BIOS is a last resort to fix a definite problem. It can stop the computer from working, if the procedure fails, so again, I wouldn't recommend this unless you have exhausted every other avenue.
Could also be a heating issue - this is very, very common with AMD chips.
I hope this helps