If you haven't already done so, Zap your G3's PRAM. . .
Start up your Mac and hold down "Command(the Apple key)+Option(the Alt key)+P+R" while it is booting (during the chime), when you hear the startup sound for the second time the operation is complete, and you can release the key combination and try installing again.
If that fails, then it's possible that one of your memory modules is bad. Try using only one of your memory modules at a time and try again.
You might also want to consider replacing the Battery on the Logic Board just for good measure. These are available at Radio Shack and macsales dot com.
Give those things a try and let me know what happens.
Good luck!!!