New Computer Randomly Reboots

I have just recently built 2 new computers, same exact components about a month ago. One of them works great, the other restarts randomly usually when installing something. The windows error files are mini.dmp and sysdata.xml and when I send the error report it tells me thats its a hardware error with either : CPU, Motherboard, RAM, or Power Supply. I have replaced all and still have the random restarts. I have run memtest86 with no errors. I've recently taken it somewhere to do a hardware diagnostics test and they've had it on for 5 days without any restarts under 100% stress loads. The only differences I can think of is that they have it connected to a different monitor and its via VGA as opposed to digital like I've had it every time. Any ideas would be greatly appreciated. Thanks