Just sounds like a dodgy earth to me as i keep saying, or weak battery/alternator isnt doing its job. on my mk2 when i had my music turned up with sub and amp the headlights used to flicker to the beat of the music, dash lights, needles everything. that was the alternator about to give up the ghost. make sure the alternator belt is properly tight, i know there on an auto tensioner but if the tensioner is weak then it wont be gripping properly.