okay, i agree with ben, sounds like a carburettor problem.
first check for false air: while engine is running, spray brake-cleaner on and at the circumference of the carb. if revs are responding on this, false air is coming in somewhere.
if not, you should dismount and clean the carb. don't worry, the xt600-carb is compared to others relativly simple. a little help you may find here:
http://www.xt600.de/xt_technisches/%...r/vergaser.htm
of course it's in german but at least you may learn something from the pics.
good luck!
mailto:klaus@xt600.de