if the idle was crap but consistent I would say dirt has got somewhere but like Bruce says it sounds like a vac hose is loose and as whatever its attached to activates it causes an air leak
grab some carb cleaner and spray it round the carby with it idleing, the idle will pick up or stall when you spray near the loose hose normally
another trick is to swap 2 and 3 plug leads, pop the top on the airfilter and give it a few good revs, normally shifts any stuck dirt
