get your power supply checked out. in the shop i work at, these symptoms happen often when a unit has a bad power supply. we have a tester that just plugs in, but for a mac, you would need to get a wiring diagram and a multimeter.

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it would depend on the Beige G3 model, one or two models used a standard ATX PSU , and a few used a Apple PSU.

the Beige G3 mobos can use both, there is a little jumper next to one of the PCI slots that is used to select what PSU is being used