Crank, usually that kind of a thing is cause by a vacuum leak. Check all the hose fittings on the carbs and intake manifold. Not just where the lines start, but the whole route. A littel wear in the throttle shaft bushings can give an eratic idle, but not noticeable at speed.
The throttle disc can hang up from closing to the normal idle position, but that would be rare if your throttle return springs are in place.
I had them hang when I replaced the throttle shaft bushings, but they broke in fairly soon.
Please let us know how it goes. pb