Extremely weird PC Problem.

[sevology]

Thanks in advance for responding. Will be glad to have any sort of input from you guys.

[liljim]

the only thing that throws a flag for me is the power supply is modular. You seem to have some pc experience and combed over most everything i would have recommended but i would maybe check the connections on the back of the power supply itself. Apart from that, you mentioned weight distribution is ok but i must be a little more specific and ask about the possibility of the cpu cooler pulling on the socket when vertical. If the cpu is being pulled away from the socket it can cause all sorts of little nasty problems. Theres obviously a connection issue somewhere if it runs on its side. If its power supply related, it will be in the connection powering the motherboard itself or possibly the vid card. Those are the connections i would trace to the psu and re-seat.