MacBook turns off while running and turns on when closed!

Dear all,

I encountered some strange phenomenon with my MacBook Core 2 Duo in the past month.

It first started with this little window which pops open when you press the power button while your computer is running. It did it on it's own without pressing any key or whatsoever. For a few weeks I was still able to click "cancel" and my MacBook would go on working as usual. But recently this window appears and even if I press "cancel" it shuts down just like you would keep the button pressed down. To be precise, it's not shutting down, it just turns off. (unsaved data gets lost!)

Now here is the strange part I: When it's off I can switch it on just by pressing the space bar or sometimes any other key?! Then it starts up, but not as it were sleeping, but rather as it were shut down propery. The strange part II: If I turn it off like supposed, it just starts up while it is closed in my backpack. And it does it all the way. So if I take it out it's just at sleeping mode!

Hope this is detailed enough but not too long to scare you guys off. :)

Any help is appreciated!

My guess: It kind of behaves like a slack joint or loose connection. Any ideas which part it could be? Power Button?

Thanks a lot!

Peer