WEIRD: COMPUTER BOOTS ONLY WHEN I RESEAT THE CPU...? HELP!

Hi,
I have this computer which won't start unless I remove the cpu and then put it back. Then it starts normally. I can reset it, that's ok, but when I power off then it doesn't boot up anymore. I can hear a sound like everything is working but nothing on the screen. I checked with different kind of memory, different kinds of psu and still problem exists. The heating is not the problem - checked that too. I tried a cpu on the other motherboard, it doesn't seem to make those problems. I think it has too do something with my mobo but this is too weird - to remove and put back cpu in order to boot!? That's out of my knowledge.

If somebody please have some idea what it could be, please let me know. I've been working on this pc for three days and still can't solve it.

This is the description of hardware:

CPU: Sempron 2800+ @ 1.60GHz
MBO: ECS 755-A2 Socket 754
RAM: 2x256MB DDR400 PC3200
HDD: 80GB Maxtor
PSU: Various from 350W to 450W Blueberry, Codegen, etc...


Please, I need some assistance on this.

Thank you.