My powermac G5 is getting a lot of screen freezes and it's hard to diagnose the cause. Sometimes the screen is just frozen (no mouse movement, no caps lock light on keyboard) and other times it freezes then I get a 50% black tint all over the screen which swipes from top to bottom and a black box in the middle of the screen with white type that says you need to restart. I can get anywhere from 2 mins of use to 2 hours before I get a crash. I've tested different drives and different systems (10.4 and 10.11). It's not system related and not the hard drive. I'm also fairly certain it's not ram. I've run the apple hardware test and it found not faults. I've taken the machine to two different apple tech's and neither could diagnose the problem. Has anyone experienced anything like this or can offer some advice on what to check or do? It's very frustrating!