My 2007 bought 2.66ghz Quad mac pro is developing a really nasty habit of crashing with no warning in the middle of working, its connected to two monitors via the stock 1900XT card (which I think is maybe the problem). Basically the screens just go blank and the keyboard becomes totally unresponsive, I have to hold down the power button to shut it down. If I try to restart it generally all the fans kick in at full speed but it wont boot and I have to leave it for about 15 minutes with the power disconnected before I can get it back up. Frequency of the crashes has gone from rare in the past few months to happening now any time the machine is on for longer that ten or 15 mins, I have looked in the system logs but cant find anything in there. I can post once the machine is back up (typing this from my MBP). I am running the machine on SL 10.6.1. Crashes predates SL though. Is it likely to be the graphics card?