Hello, maybe someone here can help me.. I have have had my iMac G5 for at least 3 years now, it is out of warranty. I have Mac OS X version 10.5.5 build 9F33, I have 512 MB of RAM (I also want to add more RAM but not sure how to do this yet). I run it with an external hard drive. My monitor just goes black and I hear the computer shut down, when I'm in the middle of something it will do this. Sometimes it was fine for a few days, then on one occasion it kept shutting down within minutes of being turned on. ? ?