My pc restarts if it is nudged or moved slightly. I have a Thermaltake soprano DX case and the heavy door also causes it. OH ya, and if i open my disc drive. it restarts .
I'd have to say just open your PC, and make sure all of the connectors are pushed down onto the mobo tightly/making contact. You should also try what Chris said, if there's no loose connections. Swap out for a different PSU, and see if that's the problem.
The loose connection might not even be in the plugs... could be between components on the inside or the wires themselves... but give another PSU a try.
I've had two cases for this... the first was actually a USB plug on the motherboard. Something was touching that shorted it out (didn't care to mess with it as I had five extra, hah, so ). Then there was a friend who put his motherboard on with no buffer from the case. It worked amazingly enough, but the slightest movement caused a reset (I wonder why ). So keep an eye out on the various metallic components, as those could be the cause (they get mad easily, when touched improperly).