I've worked with these for a while and I believe it involves a loose solder joint in the memory holders. If I warp the motherboard slightly, it boots. With it just sitting flat, it doesn't. I have to apply pressure in the middle of the board. If I warp the board and start it, and then remove the pressure, it then hangs.
I've searched many times for a resolution to this. Other then pulling off the black memory labels and reflowing solder, I can't think of any other solution. Basically it means buy a new motherboard. They aren't too expensive on ebay.
I've tried many different parts: cpu's, powerboards, memory sticks. The one common thread is that it will boot if you have the keyboard off, put your left hand under the cardbus area so as to raise that in heighth and apply pressure to the powerboard area downward.