Just had the same problem, traced it to one of the screws on the secondry carb buterfly valve coming loose and allowing air to be drawn in at idle.
my screw had disapeared completely, or at least i hoped so, but the bike came to an untimley stop about 2 weeks ago about 300k after fixing the problem. Jury is still out on why, but i should know by the end of the week.
But definatly sounds like your drawing air from somewhere.
I checked everything, eventualy went back to basics, removed the carbs and blew through them, thats when i noticed it was the secondry not the primary.