Gnx4 Volume cut problem
I will be playing along and all of a sudden the output volume will drop out like 80 % and i can barely hear my self and the tone will change then, sometimes it will go back to normal, or ill change to the left or right out put and it will go back to normal. This happens when i change banks too, sometime I'll go to bank 6 it will be screwed, so i go back to 5 then 6 again and its fine. and this also happens when i step on the expression pedal too. Help! 
Comments
A: dirty/dusty master vol pots on the rear panel
B: dirty/dusty area around the exp pedal.
use some canned air or an air compressor at about 40-50 psi or so to blow out all the dust debris.
If you tried a facty reset and exp pedal calibration previous to cleaning it up you may have to do the procedure again.