Ok, back again and the last night I hope. In the A3k case there are other spacers then in the A4k. The card should working also without these spacers. If everything is running fine once can fix the card with 2 screws. That's not the problem.
I pressed the card a little at different positions and found an interesting electronic part. Dont know exactly what it is, but maybe a temperature fuse. If I do some pressure on this element the card is booting. But it needs decent pressure. So the card is twisting a little and the contact problem could be elsewhere.
But anyhow .. the magic part is labeled with "150 PU90" and it's made of two metal parts on top of each other. It is in the near of the voltage regulator and a little diode is direct connected labeled "PH-".
So the card has a problem, because, if I put my finger on this metal all the time it boots up, so my hardware config is ok. But I have only 2 hands and need them both :-)
How can I post a image ? Then I could show the special part.