Onyx 550 Powerbook shuts down right away

[Solarteur]

Hi,
I have a 550Mhz Titanium Powerbook which has weird starting problems. In 30 out of 100 start attempts nothing happens when I press the start button. In 50 out of 100 start attempts it turns off right after the chime ( before any Video activity). In 10% it stays powered on with a grey screen after the chime. In about 5% it shuts down during OSX boot and only during 5% of all starts it boots successfully.
When it boots into OSX 10.3.9 it falls asleep randomly after a few minutes. When I boot into OS 9.2.2 it runs flawlessly.
Here are a few things I have tried:

Reset P-RAM, NV-RAM and PMU
Switch RAM with other Ti Powerbook
Switch AC adapter and Battery with other Powerbook
12 hours Hardware Test looping (196 loops) without error

Any other suggestions except switching the logic board?
Thanks a lot for any hints.