I received a 2008 mac pro 3,1 in pieces from a friend . He said that he was having trouble with the graphics cards cutting in and out and and then the power supply (PS) failed. I got a used power supply on ebay and put it all back together. I turns on, the graphic card ati 3870 (GC) fan powers up but i get no signal to the monitor. I tried all my monitors and nothing. I tried several different OS (10.6 , 10.7 and 10.11 ) versions, I moved the GC to all the different PCI slots, i pulled the GC from my 2006, (an ati 5770), which i know is compatible and still the same results. It boots up, 'With LION OS 10.7 I booted in safe mode and blindly typed in fsck -fy command and waited, then typed reboot and it rebooted. It will boot to target mode and i can access the drive in disk utility and verify it. I have done pram, and smc resets, I tried the old PS and it fired up as well, I then did all of the above with the old power supply back in it. I have been reading forums and post for days and can not find any more info other than it is possible it is logic board? Ideas or advice would be appreciated.