MacBook Pro won't shut down - keeps restarting

Hello,

my MBP (2009 2,66GHZ) is showing a very strange behaviour.

Whenever I shut it down (Power button or Apple menue) it shuts down fine and once it is powered down immediatly starts up again.

First I thought the power button was stuck or the keyboard cable not properly inserted, but even with the keyboard disconnected it keeps doing it.

When running there it performs flawlessly.

- SMC or VRAM reset has not led to any change.

- I switched the MB for one that is knowingly good and the computer worked fine, so it is some issue related to the logic board.

Has anyone got an idea as to what the problem might be related to?

I appreciate any answers.

BR

!BATTA!