I suspect the ground connection of your amp. You get 'lifted' when you touch the mic because the shell of the mic is grounded (and you definitely are not
This can be lethal!!!
I'm going to assume you're more artistic than technical. Also other folks reading this may not know anything about this stuff, so please, bear with me.
Most guitars come from the factory with the bridge or tailpiece tied to the shell of the output jack. This provides a ground connection from the strings, via the the jack cord, to the amplifier chassis. In most situations, this is a nice, safe, quiet setup.
The GND switch (usually installed next to the amplifier power cord), connects a small capacitor from Ground to one of the AC power lines. Usually (not always), the amp will be quieter when this capacitor is connected to the Hot line. Sometimes, connecting to the Neutral works better. But then again,
somtimes it doesn't work at all...
First, inspect the power cord, and the plug.
If the cord has been stepped on too often, or pinched under some heavy equipment, it may be shorting the Ground line to one of the power lines. Another common problem is the fitting where the cord enters the chassis. If the cord has been bent sharply at this point, it may be shorting internally.
Examine the plug closely. Molded-on plugs, in particular, can be damaged internally if they have been pulled out by the cord. It may look cool to yank on the cord five feet away from the socket, but how cool is it to electrocute yourself? If the plug is the type that can be taken apart, inspect the connections. A small stray strand of wire can cause big trouble.
If the plug and the power cord are OK, its time to see your favourite tech. Things get a little trickier, and a lot
more dangerous from here. I'm going to explain some more of the trouble-shooting process so you can have an intelligent discussion with your tech. Don't try this at home, folks!
The Ground switch is the next thing to be checked. My guess is that the switch is broken internally, and is not actually disconnecting the capaciter when you move it to the 'neutral' setting. (Not to be confused with the Neutral line of the AC power).
Then the capacitor. I call this a 'small' capacitor because it has a numerically small value of capacitance. In this case, the low value is important because a small capacitance will conduct high frequencies better than low ones. The 50/60 Hz AC power frequency is considered to be low, compared to the transient spikes that can make clicks and pops.
A partial failure, known as a 'leaky' capacitor, allows too much current to flow. In your case, this would mean that the 'ground' of your amp, and your guitar, (and you!
) would actually be 'hot' relative to a 'good' ground (like the mic and it's stand).
There are other points in the circuit that can cause this problem (like cracked insulation in the power transformer or any of the high-voltage wiring), but the capacitor and your Ground switch are the first items to check.
*** Please take your amp to a qualified tech. This stuff can kill you. ***