From what I've seen over years of reading computer and video card forums, there are three common reasons for this:
1) Card is failing. Replace card.
2) Card is over heating. Fixes include blowing out the dust, replacing the stock sinks/fans with improved aftermarket cooler, removing the heatsink and replacing the thermal compound, and improving airflow in the case.
3) Bad drivers, solved by a driver update (but I've only seen this about Windows, not OS X).
Just had my Mac Pro freeze with all little squares right across the screen, turned it off and re booted and all OK, but I assume this is the first sign of my graphics card folding, or could it be a over heating problem ?
If the fans and heatsink in the MP and the card are clear, be prepared for it happening again soon. I would make plans anticipating it going wrong again by looking around for a replacement.