Mac pro 3,1 8 core, hangs at bootup, at the same spot

Ived tried reseting pram, smc,
ive tried safe mode

ive tried severall different boot disks(ssd, 2.5 and 3.5 hdd, all different manufactureers)

ive tried 3 different OSs (10.6.8, 10.7.4, 10.8)
ive tried 2 different gpus (8800gt and ati 3xxx)

ive tried 2 different sets of ram on two different ram cards
ive tried dissconnecting everything

Help please im out of ideas and i really do not want to have to fork out the big bucks for a new computer right now

when i try booting in 10.6, it got to the desktop, but then locked up right away, before the keyboard even worked
in lion it got to a screen for me to put in my password to "resume" my current session, but then locked up, again no keyboard action

mountain lion just gets me to a gray screen with nothing on it

sometimes instead of any of those, i just get to the gear at it locka up with the gear stuck in its postion