Try re-seating the RAM. I've seen PCs do what you describe when the RAM's contacts aren't clean or are not seated properly. If that doesn't do it, then disconnect everything and reboot, each time adding one device/cable back into the mix, until you discover what is causing the problem.
I also ran into a problem like you describe when I had the power for my USB DVD drive wired up with the center of the power plug wired for positive. Turns out it needed center negative.
You'll figure it out via the process of elimination. Good luck.