Hi! Do you have an Airport installed in this machine? The reason I'm asking is that I've often found that if the metal Airport antenna plug is left loose and dangling, and the memory shield is not installed, it can short out the computer by touching the memory or other components on the logic board. Probably an unlikely scenario in your case, but something to look out for.
I would also turn the computer upside down and shake it lightly, and see if you hear loose screws. Loose screws bouncing around inside the case can also short out the board. I've been sold many a "freezing" iBook only to discover the laptop was fine, but that there were loose screws inside causing it to freeze occasionally.