Video fans stop,video stops: computer stays on.

I have 2008,A1186 EMC 2180 3,1 Mac Pro 12 gb ram running OSX 10.9.5.

The unit ran great for several months using two 27" monitors; the video started shutting down gradually until it stopped altogether. I replaced the original ATI 2600 with an NVIDEA gtx 8800 GT. Nada at first; so I installed in slot 2; that worked, but I could only run one monitor. Two monitors would stop the fans and thus the video.

When I switch back to the single monitor, the unit will shut down the same unless I leave it alone for a few days.

After that, it will stay on for hours, sleep, shut down ,reboot, and act normal.

I bought this Mac to run Ableton and other DAWS, but because it has become so unreliable, I'm using a late '12 mac mini, which is good, but can't touch the '08 Pro on speed and quality.

Can you kindly assist me; I have upgraded both my '09 mini and the '12 mini; though I'm aware of the complexity of what I may encounter, I am confident that I can tackle it.

Where do I start?