My MacBook Pro randomly logs me out and in. HELP!

My MacBook Pro 3.1 (late'07) is randomly logging me off when it is under load. Like running Lightroom, Safari stuff like that.

It also happens, when the MBP is running for a couple of hours and I play a video in VLC or Safari and maximize it.

The display turns all of the sudden blue (the blue you see, when you start your Mac), logs me off and instantly on.

All Programs are closed, no work is secured.

It's like his problem: http://www.youtube.com/watch?v=vgCccEXLsrE

But mine occurs more randomly

In another forum someone said that the WindowServer could be the problem. But I can't find any solution.

Reinstalling the system did not change anything

Another symptom was also a flickering display (both, the external and the build in) and a total freeze.

I don't think that it runs to hot because the MacBook is on a stand and it has also happened in winter.

The Logic board has been exchanged twice because of the Nvidea GPU failure.

Please Help!