Brian,
I hope it is just a matter of carburation, as that is easy to fix.
Other possibilities, and not quite so easy to fix are:
Uneven timing; I have seen badly machined end covers for Lucas magnetos where the cam ring was not concentric to the armature. The result was the points broke sooner for one cylinder than the other, and resulted in low speed vibration that would smooth out slightly (or was masked) at speed. Usually it is easier to just modify the cam ring to get even timing.
Bad crankpins/rollers. When my professional (expensive) rebuilt Mk3 crank started to fail, it started to run rough and vibrate; worse under load. Sort of like the sensation of taking a hill with out retarding the spark but the engine is not quite to the point of pinking. On disassembly the front crankpin bungs had fallen out, starved the unit of pressure lubrication (it was surviving on splash), and the roughness was from the flats worn on the rollers.
Crankshaft twisted/shift. This I would think would vibrate worse the faster the engine turned. I have not had it happen directly to me, yet.
I repeat, I hope it is just unbalanced carbs!
-Doug