I purchased a 5,1 2x2.4GHz least week into which I've transplanted 2x X5680. When I purchased they machine the previous owner had an Nvidia 460 SE installed as they said the original ATI card died. The owner mentioned that they were having problems with the machine not booting and the gpu fans running full speed. They said they found that the GPU wasn't seated properly and the metal bar restraining the GPU was loose. In front of me the owner started the machine a few times and it seemed all good. I figured that was the end of that (460 being a short card, perhaps that's why it wasn't seating properly.) Got the machine home and cleaned out a lot of dust from it with a compressor. Replaced CPUs with X5680s and she seemed ok. Replaced the 460 SE with an ATI 4870 from my 2009 Mac Pro. In the past week I've had 2 instances where the machine won't POST. This evening I installed a HDD into caddy #2. When I turned machine on it wouldn't POST. In order to get the machine to boot I removed the GPU, reseated it and tried again. No post. SMC reset, same. Pressed RTC reset on backplane and then she booted. It seems like this machine has a finicky backplane where if a little pressure is applied to it, she fails to POST and GPU fan runs full. I ran AHT and I get a PCIe temp sensor error which apparently is relates to the 5,1 firmware not reading the 4870 temp properly. Extended test gave a 4hdd/11/4000004 sata (4,0) error which i presume is a HDD failing. All I can think of is that the backplane board is loose or not sitting quite correctly. Has anyone experienced this or know where I should begin? I was thinking contact cleaner on PCIe slots as a start. The backplane to processor board connector looks good. Could a loose connection between PS and backplane cause a failure to POST and GPU fan to run full blast with a black screen? Any suggestions welcomed. I hate to think I was ripped off. When running the machine is fairly stable. Have only has a single KP.