Yes, it could be many different things. For people with mobos that don't have the ability to lock the PCI and AGP buses at 33/66, 120FSB can be a problem due to the PCI bus being too fast or too slow, since it'd be at 40MHz using a 1/3 divisor and 30MHz using the 1/4 option. Of course the BD7-II has the option to lock these at their default speeds, regardless of FSB, so I recommend you use that setting (if you aren't already) for stability by eliminating this potential variable.
If you haven't already tried increasing the CPU voltage, yes, you're likely at a point where it'll require additional vcore to remain stable at higher speeds. With more CPU voltage, the chip will generate additional heat, so keep an eye on your temps. The P4 automatically throttles back on speed once it reaches ~63'C, so you never want to hit that temp when under max load, or your performance will be adversly affected. Now it probably won't require much more voltage for stability at just 120FSB, so just try .05 or 0.1 more and see if that is enough.
Your mobo also allows for up to 2.7 volts on the memory, which could be considered the "safe" limit for just about all DDR available. If you're attempting to use either of the previously discussed cpu:mem ratios, you'll probably need to increasse vmem too. In your case I'd suggest using the 2.7v setting regardless, as I'm confident it won't damage anything and will likely help in your situation. Also, the instability could be caused by the settings used for the memory timings. If they're currently at 2-5-2-2, try changing them to 2.5-6-3-3 for improved stability if memory is suspected as an issue.
Though I suppose it's possible, it's not very probable that WinXP would be your issue. What do you mean by "it runs randomly"? Does it lock up during boot, are you getting lockups while attempting to run benchmarks, do the apps simply kick you back to the desktop without locking up the system, are you getting BSOD's with specific WinXP error messages or what? Try the things I've mentioned above and see if you're able to get it stable at 120FSB. BTW, is your CPU a Williamette or Northwood?
