Ask Max: PowerMac G5 crashes to black screen and full fan speed in Safari
My G5 running Safari frequently turns all the fans in the case to max speed and my monitor goes black (no message displayed) and it acts like the Mac is trying to shut itself down. Had it to two Apple repair centers and neither could make it act up and they could not find anything wrong with it. Last place changed my power setting lower. There must be a solution to this issue.
It is interesting that Safari alone is causing the problem. I would first check to see if browsing the Internet in another application creates the same issue. Try downloading the Firefox browser and use it instead of Safari for a while. If the problem does not occur with Firefox, then throw out your version of Safari and get a new version. A good way of getting a fresh version is to download the Combo update for your Mac OS and install it. The combo update should have Safari. If a fresh version of Safari still causes the crash, then try using it from a different user account. You should create a new user account just for this test. Only use it for testing. The problem will not happen in the other user account if a setting of yours causes it. If you find the crash is caused by one of your settings, delete the Safari Folder in your user accounts Library and the "com.apple.Safari.plist" file in your preferences folder.
It is altogether possible that the crashes are independent of Safari. Some G5 towers need to be thermally calibrated, but that is not a likely cause. Calibration work can be performed at an Apple Authorized Service Center. More likely it is a RAM issue. If there was extra RAM installed, try removing it and see if the problem continues. Remember RAM is installed in pairs so remove two sticks at a time from the same numbers slot. The RAM in slots labeled 1 is most likely from Apple but try just having one pair of RAM, stick in at a time for testing, and then test another pair.