Same setup for my problem. It won't boot at all with two memory modules installed, but boots up and has been running solidly for several days now with only one module installed (in the upper slot.) Have you tried this? Obviously, running on half the rated RAM is no long-term fix, but it would point to where the problem is.
Yup, messing around with this did it. Turns out it will boot with one of the DIMMs in the top slot, but not the other DIMM. So I guess somehow a bad memory module?
Regardless, it's booting now. Thanks for the tip!