HP Laptop Won't Boot

I have a laptop that is proving more and more obnoxious to troubleshoot.
The first problem it exhibited was a BSOD on startup: 0x50. I could not boot into safe mode even.
I read up a bit and it seemed to likely either be a RAM or HD issue. I booted from a Vista DVD to run a chkdsk. Every time I selected the partition for repair though, the computer would just turn off.
I booted from Linux and the HD seemed to be fine. I also popped it into a USB dock connected to my Windows 7 desktop, and it also appeared to be fine.
Next I ran MemTest86+ via USB on the laptop. The computer would simply turn off one to five minutes into the first pass.
I tried each DIMM separately with the same result. I then popped a separate DIMM, which I know works fine in my netbook, with the same result.
Just to indulge I also popped the HD from my netbook w/ an XP installation on it. The computer just restarts a few seconds after the XP splash screen pops up.
I also have run the stock start up diagnostics on the HD and RAM. It will successfully complete...sometimes. Usually, at no particular point within the diagnostic the computer just shuts down.
At this point overheating is worrying me a little, since it seems the longer I am working on it in one sitting, the less time there is between shutdown. I have dusted around the HD, RAM, CPU and exhaust grates, but have not taken off the entire bottom for further cleaning yet.

The specs of the laptop:
hp pavilion model dv5-1159se - 3 years old
Vista x64 OEM - 3 years old, original installation
Microprocessor - 2.00 GHz Intel Centrino Processor Technology featuring Intel Core 2 Duo Processor T5800
Microprocessor Cache - 2 MB L2 Cache
Memory - DDR2 4096 MB
Video Graphics - NVIDIA GeForce 9200M GS NB9M-GE
Video Memory - Up to 2047MB (256MB dedicated)
Hard Drive - WDC 320 GB (5400 rpm)

Since you tried known good ram and it failed, motherboard swap is next.

That's all I can think of that can be the issue with your symptoms. It may be a power supply or heat issue, but you'd have to take off and re-attach the heatsinks with new thermal paste. Can try a BIOS update, but with the laptop crashing randomly that's really risky and low chance of fixing anything anyway.