Motherboard problem or something else?

I've got a new Gigabyte 7VAXP, a GeForce ti 4200, Athlon XP 1800+, and pc-2700 2-2-2 mushkin. If i use the optimized or high performance settings in the bios (you can't set the individual values like agp bus or memory latency) I get restarts when using semi memory intensive programs (or the sisoft memory burn-in). if attempt to set my cpu's clock at the actual speed with anything more than the fail-safe defaults, the computer won't even POST. I've used just one dimm, then just the other, but same results. I've updated all applicable bioses and installed the via 4-1 patches, etc, but nothing works. Do I need to RMA my motherboard or something else? please help.

thanks

antec true 430 and i don't have nearly enough stuff crammed in there to need bigger.