something loose?

heres my problem. When I have the computer in an upright position, the computer wont turn on some of the times. The HSF will spin for 1 second and thats it.. Thought the Mobo light will always be on. Now when I have the computer on its side, the computer has no problem turning on. All the connections seem ok. the mother board is not completely tight cause I know sometimes that will cause the comp not to start. any ideas> ?

sounds like you have a wire or something shorting out. Try a different power supply. Make sure there's no wires dangling around or stuck behind the mb.

Maybe your fan is stuck, which causes the CPU to overheat (because the heatsink can't dissipate the heat fast enough) and crash.
Well that's my theory anyway

if the fan was stuck then why would it work lying down? that doesn't hold water. Most reasonable explanation is something that moves when it's lying down

Sounds to me like a short between the mobo and the case. Use either plastic stand-offs to support your board in the case, or use fiber pads between the screw and the mobo.. Double check all power connectors, try wiggiling the wires when it does work to see if you can find out which one it is..

Good luck