Today I realized that I had purchased PC2700 and my processor only
requires PC2100. Is this causing my problem? My research leads me to
believe that the board should be perfectly happy with the memory
running asynchronously at 166 or underclocked at 133, but maybe the
onboard video isn't as forgiving.

I'm not sure by your wording if you have actually tried underclocking the
PC2700 yet or not. My guess would be that you haven't.

I know on my Asus A7N8X Deluxe the memory frequency MUST be syncronized with
the FSB or I am bound to run into stability problems. The Asus group is
almost always flooded with memory questions that lead back to incorrect
memory/fsb frequency settings. I'm pretty sure this is related more to the
design of the nforce2 chipset than just the Asus implementation of it.

So my point is that if you haven't tried synchronizing the memory with the
FSB, then doing so should take care of your stability problem.

--
LiveWire