I have a 17" iMac G5 out in my garage I use to surf when I am working on things outside and lately it has been going to a blank screen with rushing fans after it has been on for several hours. I cleaned the lower CPU fan, checked the capacitors, basic inspection and cleaning revealed no obvious issues. After unplugging the unresponsive computer and plugging it back in it seems to run fine for an hour or sometimes several hours before going back to a blank screen and full speed fans. What system tests can I perform to determine what may be the issue? ~TIA.