Do you know what you're talking about? I am convinced that you do not. Isolating the ground will do nothing to YOU assuming you don't fawking touch live wires and/or circuitry inside the computer, which you would not be doing anyway.
Just to clear up YOUR misconceptions, the CRT ground lug is used when discharging the tube itself. http://apple.doit.wisc.edu/servicesource/ss/safety/safety.html#Discharge
OP - That is certainly an odd situation you have going on there. So you have ruled out the house wiring by testing it at your parents house.
Just to be clear, only one of the Pluses trips your RCD correct? Not all of them? The other six just lose video but stay powered on? I think you should pick one and carefully test the analog board. Touch up all of the solder joints if you want.