Source : XLR8yourMacA number of Mac Pros, all fitted with the Radeon X2600 XT, suffer a similar problem to that seen by owners of the iMac alu - also fitted with the Radeon 2600.
At random times, the screen freezes and the machine grinds to stop making all work impossible.
The exchange of the graphic boards does not solve the problem in all cases, on this Apple has still to comment. None the last updates - eagerly awaited especially those that had some bearing on the graphic components - brought a solution or an improvement. An online petition has been set up with the goal to force Apple to fix this problem
http://www.ipetitions.com/petition/nomorefreezeonMacPro/
Note: only machines fitted with the Geforce 8800 GT seem affected by this bug.
[translation by cr