won't power on, I just smack the machine and then it powersup

There's a specific issue with the G5 tower having a tendency to fail due to solder problems on the lead-free machines; the ones with leaded solder never had an issue.

It's the chip with the massive heatsink that goes bad on these, but the best practice is to treat both as bad because one going out is not so great; the other one tends to go out next. However, this is a TEMPORARY SOLUTION and not permanent. For a longer-term solution, you will need to replace the motherboard and ASD; you can look online for it, but I'm not providing a link (COUGH Macintosh Repository COUGH) once you calibrate the processors AND replace the motherboard that fixes the problem.