Screen corruption - faulty hardware?

Hello all, I'm currently suffering from the following problem: At a certain point in time, the screen will start to shake a little bit, becoming worse, and then start to corrupt, and will eventually lock up. This can take as little as a few seconds to a few minutes. This is independent of the OS. I've tested the RAM, have tried it with a different PSU, and a different graphics card: no luck so far. Any idea what might cause this? Regards, Pieter Hulshoff