I use to have the same issue. Changed PU, pots, shielding etc. No improvement, no cure... I thoght. At one day I had to fix the electric connectors in the wall. Hey, the ground wire was damaged! It's an old house with some broken cables in the wall. Yes! There are some connectors wich cannot be grounded anyway, but some were fixed. And the issue is gone. 
Our guitarist has the same issue with his amp, but the ground is disconnected somwhere inside the amp. Well, we connected our amps by a "crock" (two EQ buttons... LOL), and voila...
Hope this helps...
Our guitarist has the same issue with his amp, but the ground is disconnected somwhere inside the amp. Well, we connected our amps by a "crock" (two EQ buttons... LOL), and voila...
Hope this helps...
