OK, you've eliminated the motherboard by replacing it, and the video card by swapping it with another. The power supply is still suspect as you can not test via software alone, and the voltage monitors in BIOS are notoriously inaccurate. You need a calibrated test rig to properly test, or, swap in another power supply.
This leaves memory. Download and burn to disk memtest86 ( http://www.memtest86.com/
). Run no less than 3 complete cycles on each stick of RAM individually in each slot. Follow this by pairs, alternating slots between tests (it's gonna take a while).