Originally posted by: isosunrise
My problem is, anything at 2.8GHz (7x400, Mem:FSB 1:1) or higher,
You've already figured out the cause right there. It was one of the major reasons I gave up on C2Q's. Decent boards will handle up to 450FSB but anything past 400FSB or even lower for budget boards didn't give me a rock solid feeling. (By rock-solid, I mean something that'll last 2+ years without trouble) I've tested total 5 different boards including Intel's own X38 board. In order to fix that, not only the board should be capable but also there should be right voltage choices available in BIOS. And good cooling on MCH as well as all circuits around the CPU socket.
Then the fun begins - vCore, vCore refrence, vMCH, vMCH reference, vFSB (there are two of them since these are MCM chips), vTT, vTT reference, vPLL, vDIMM, vDIMM reference,.. you get the point. The individual dies (Wolfdale) are not even close to their limit but you have to spend most of testing time finding 'just right' voltages for FSB, or high FSB will eventually result in funky behaviors. Your case is further complicated because of IGP on G31, even if you don't use it. There is a reason why Intel had to deploy FBDIMMs for server CPUs. (it dramatically reduces load on MCH, thus on FSB/motherboards.)
It's stupid because all these could have been avoided on single-socket systems, had Intel given us decent multipliers. (at least x9, ideally x10 or higher)