distortion problems

Whenever I play with a good amount of distortion I notice that strumming the high strings (e, b and g)results in those strings sounding extremely shrill, broken up, and out of tune. Essentially, the sound I get from playing these strings is little more than unstructured noise. The problem is especially bad when playing barre chords. I usually deal with it by avoiding the high strings, but this limits what I can play. Is the problem most likely with my guitar, my amp (which is pretty beat up), or me?

Try changing your tone control settings. What works well for a 'clean' tone will likely be all wrong when the going gets hot'n'heavy. Start with the classic metal 'scooped-mids' setup, (max bass, min mid, max treble), and play around with the settings 'til you like what you hear.