I use Gibsons and a marshall stack. And you simply cant stand too close to the amp without getting funny noises. Feedback is a natural occurance that you have to learn to control.
I can it "phasing out" when the notes turn all mushy and zap outa tune. Try muting strings 2 to 6, and then picking the 17th fret of the first string, does it still go to mush? If it doesnt welcome to the one going battle of tone versus gain :(