MacBook doesn't boot, shuts down after releasing power button

My Macbook pro easrly 2008 2.5Ghz with the 512MB Nvidea chip has stopped working.

1. - When I push the powerbutton, the system tries to startup, but immediately shuts down after releasing the powerbutton. This happens with all three options of the AC adapter, battery and AC + Battery connected.

2. - When I push and keep down the powerbutton the system tries to startup, but shuts down randomly. Sometimes I hear the DVD drive click, but the system shuts down after a while (timing is different everytime I try) normally shutting down is preceded by the SIL (Sleep Indicator Light) blinking very rapidly for a short period. This behaviour is the same wether or not I try with the powerbutton pressed down, or released after hearing the DVD drive clicking.

2a. - Immediately trying to power on the system again results in a very quick restart (but fails within 1sec) or the system starting up itself after not being touched for a few sec.

3. - Removing both DIMMs from their RAM slots starts up perfectly with just a press on the powerbutton and gives me the usual result as would on any Mac. System beep every x seconds.

4. - Reinstalling 1 of the DIMM's in either RAM slot gives exactly the same behaviour as in 1. and 2.

I have already reset the PMC (hold powerbutton for 10s) several times. With RAM installed and without. As well as resetting PRAM (startup with command-option-P-R).

If anybody has a clue as to a solution or fix I would appreciate it a lot :-)

Many thanks in advance!