ground issue / Impedance

Hi mate

I hate seeing unanswered threads so I guess your options are either test your G8 at another location (friends house?) to confirm you can replicate the feedback problem if you can and you've got a firm understanding of electronics I suppose you could get hold of a decent multimeter & the engineers service manual for a G8 (if such a manual exists) open it up, locate the ground circuit and test the wires for either too much or too little resistance which should help you identify the bad earth.

Or

Call your local Roland Dealer and organise a repair.

Sounds to me like a grounding issue.
Also - are you using balanced or unbalanced cables? If your amp / mixer has balanced inputs, use balanced cables, or get a DI box.
Also, if it's a grounding issue, a DI box might help, with it's "groud lift" function.
Btw - if you're in Europe, you can easily sepparate the grounding prong from the power plug, to test if it's a ground loop causing you all the trouble : just use some scotch tape to isolate the copper connectors that are located on the side of the power plug, and plug it into the outlet. If it was ground related, the noise should have disappeared. In which case, get a DI box, or good balanced cables.

IMPORTANT : do NOT leave/forget the keyboard with the ground prong isolated - while it's safe to do so for a limited time, when testing things, using it like so regularly can result in electrical discharges that could lead to hardware failure, injury or death. So use the testing method i described above just for that - TESTING. if it proves to work, don't leave it like that, but try to find solutions instead.