Dimension 5150 start for about 2-3 sec. and then stop

 I have been strugling with my Dimension 5150 for 2 weeks now, it start up, run for 2-3 sec. and then stop - there is a green light on the MOBO all the time, also efter the PC stop, at start the push botton light up, first orange then green, the four other ligths on the front panel also lights up.
The fan for both CPU and power supply are running at the start for the mentioned 2-3 sec.
I have testet all the RAM, one by one - no problems there.
The BIOS have been reset.
Battery replaced.
New (other) grafic card mounted.
All other cards removed included Hard Disk.
Further I found the the heat zink on the processor was dried out and put new heat zink on.
There are no beep at start, nothing at all.
I' running Windows XP SP3, all updated.
I have 4 GB ram mounted.

Now, for a couple of days ago I desided to give it a second chance, I stripped the PC, took of the MOBO to see if there should be any bad soldering but nothing found, then I put it all together, still nothing, then I removed the flat cable there goes from the MOBO to the front panel, the miracle took place, the PC started up.
The funny thing is, now that I only can start it by means of the power supply that it takes some second before it start.
After using the PC and turn it of I have to wait ½ hour befor I can start the PC again?

Apparantly there seems to be an error either at the front panel or the connection to the MOBO, but considering the simple layout of the front panel its hard to believe that it should cause the problem.

Does anyone have any similar experience with the front panel?

Best regards,

icepirat