i might just bite the bullet and get a new motherboard and see if that helps at all.... i hate this type of problem, so hard to trace without replacing components.
You and me both, part of my job is sorting out systems that others cannot fix in the field, luckily I have a stores full of new components to play with which makes life considerably easier. Motherboards are the worst to diagnose, especially when a faulty one can take out a processor or memory...unless you realise whats going on you can lose a lot of components that way.
Best of luck
Heh.. I used to be in the computer hardware/assembly business, so I know exactly what you mean. I once blew a couple of processors (Cyrix MIIs) without realising that I'd set the mobo voltage for Intel chips.
Funnily enough, the one machine I'd assembled for myself ran just fine, although it' heat up every now and then and restart.
This reminds me. I need to clean my computer. Dust once killed a DIMM slot on my mobo as well as the memory. Ugh. I hate dust.