T5500 Second Processor Riser Board Problem

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Hi, all.

I have been a longtime fan of the Dell Precision T3500 (Socket 1366), having run one as my main rig for the past five or six years now. It has done everything I have asked of it and it is still fabulous. I didn't particularly need to upgrade from it, but the bug bit me and so I bought a T5500 motherboard and installed it into a spare T3500 case I have. Combined with a T5500 wiring harness and a 1000w T7400 power supply, it all went together and functions perfectly... as a single processor machine.

But a T5500 with one processor is pretty much just a T3500, and so I ordered in the appropriate riser board and installed it along with a matching processor and RAM. Plugged everything up and went to start it up. It POSTs okay, but it keeps throwing an error about something being wrong with one of the DIMMs on the riser, and it seems specific to SLOT 2 and not a particular stick of RAM. Looked at it and cleaned it a little bit. Still with the RAM error. But that is not the big problem. I hit F1 and it continues to boot normally, then it starts to load the OS and suddenly, the power is cut. <click>, and off.

I don't think it is thermal, because I can run the entire pre-boot memory test without issue and it will idle indefinitely at the BIOS screen, but the first time it tries to load Windows or Linux off of either a hard drive or a flash drive, it gets to the same point in loading the OS and just shuts off.

Any ideas?

Thanks