Humming And Voltage Through Guitar

I have a problem that my gear hums like a jet. I have Seymor D's JM and JB combination in my custom made Anders Liljeström les paul model (further detail check liljestromguitars.com), it's been checked and it is grounded so there shouldn't be a problem. My amp is Crate's V3112.

Problem is that the whole thing (amp+plug cord+guitar) has quite nasty voltage going through, it pinches like if you put your tongue in 9V battery. Don't need to try to get a catch from what I mean.. Anyway, I think that the whole thing is a one big "antenna" for distress signals from other eletronical devices and for example moving my finger on a plug makes the amp rattle.

How is it fixable? For what I know, I can't get my guitar grounded multiple times? Though it doesn't bother me much when I play (the circuit is grounded through me when I touch it), when I take my hands off the guitar, it starts humming and rattling; open chords don't have sustain at all if I loose my grip. What can I do?

I've tested other amps, other guitars in my amp and other guitarists seem to have this issue too, but only in smaller context.