Weired PC problems...

I'm currently receiving very weired problems with my workstation. When I switch on the power supply the HD and CD-ROM react normally and start on, after that nothing happens for a while (about 5-10 minutes), no screen output, no more reaction on the periphery. Then suddenly the maschine starts booting (normal graphic card initialization, RAM diagnostig and stuff).

I currently have no clue what's going wrong. First I thought it might be a problem with my graphic card, but inserting another one doesn't solve the problem. Detaching the HDs and CD-ROM did not manage that either. Even detaching the RAM did not work. My current assumption is, that there must be a fault on the mainboard. :(

Maybe someone has a tip on what else can be wrong? Or how to fix it?