My initial reaction was a cable hanging up, but since you've checked that, I'd think air leak.
I had that going on Saturday briefly followed by the bike stalling when coming to a stop. I suspected an air leak, too. I replaced my carb-to-engine rubbers and that fixed it. Are your rubbers on both sides of the carbs in good shape?
I haven't tried it, but I've heard while the bike's running, you can spray WD40 on the carb boots. If it causes the bike to run better/more normal, you've likely found the culprit.