At home I can't get past the gray screen and it shuts down.
My home location is 118 volts, Mac Store voltage is 124 volts. It boots all the way at the Mac Store every time on three different visits but not at home.
It will boot at home once after a visit to the Mac Store but shut down sometime during that day's work or just not boot the next time.

Have you tried booting from the OS disk to see if it will start up? If so, it would eliminate the power from being an issue.

I would love to know how you measured the voltage at the Store, let alone at home? Anyways, if you think that this might be the problem, Get a UPS unit. You do not have to spend a fortune on one and most units will insure that you get the right amount of power to drive your unit.

yep, agreed a UPS will stabilise the voltage at something close to what your mac needs. However, your G5 should run on 110volts RMS in the US. When you booted it in the store, was it with all your periphs connected? If not, try booting it with no kbd, mse etc.

richardtj has a good idea about disconnecting all the extra peripherals you have on the mac. There were times when I had several items (scanner, printer, etc) connected and the computer didn't want to start unless they were disconnected at start. This usually happened when these items were connected using a USB hub.

I have got a G5 and its not booting from the CD or the drive and won't see anyonther hdd what do you think it is.I am a pc computer person so this prob is new to me.