What could be a reason for movement-dependent shutdown?

Hi everyone,

here is my situation: my MBP (pre-unibody) shuts itself off - well, not randomly, but it seemed like that for a long time. At first, I though it's only overheating, so I took it apart, cleaned the fans and heatsink, reapplied new (apparently also better) thermal paste on the cpu and gpu. The temp of the computer has of course dropped considerably but it didn't help either and shutdowns even occured with CPU temps in the vicinity of 50°C. Then I tried to run the computer with a new(er) battery, without battery and on battery only - it happend every time.

In the end, though a lot of trial and error, I've discovered that the computer will work just fine as long as the top case is not screwed in anywhere and is actually not really even in its place (I basically but a little piece of wood on the casing of the battery to keep it about one centimeter higher than where it would normally be), I use an external keyboard and mouse and do not move the laptop or adjust the display or, well, do nothing physically to it at all.

Also, when the computer was still put together, I've noticed that shutdowns more often occured when I was typing on the left side of the keyboard.

Anyway, it seems to me that this might be some kind of contact fault or something like that - maybe a cable is broken, or something maybe near the I/O board - I just don't know what kind of cable would cause a complete shutdown - it's like a complete power failure - only I've already tried replacing the cable that connects the battery and the logic board and that didn't help either. So, I'm really desperate now: what the ^!*$ could it be?