I have the same problem.
I checked with others BL-5J battery and my phone start normal..
so I thought its a my battery problem.. but I have check my battery on other phone(Nokia 5230).. and its working normal..
Its really very strange problem.. any one can help on this issue?
I did not find any solution to that problem, my mobile flex cable is new. I have checked with my other n900, and that flex works fine. And swapped the display and the display works fine with my other n900. And the prob is with the mainboard.