Vibration is Citabria

We have a '67 KCAB that has a weird vibration from time to time. You can
feel it in the stick and seat. It is the worst around 2200-2400 rpm but
only in the air. On the ground it isn't noticeable. Oil analysis is fine
and the engine seems smooth. We've installed new motor mounts and
everything else on the airframe seems tight. One suggestion, from a local
KCAB expert, is that it is from the crossover exhaust. We've considered
having the prop balanced but figure that if it was a prop problem it would
occur all the time. Maybe a sticky valve but I would expect a sticky valve
to be 'cold engine' problem.

Any ideas?

Tim
N2606G