My experience indicates that when roughness occurs at engine RPM lower than normal operating, and as evidenced by your comments and the excessive mag drop, the engine magnetos may be the cause. You make no comment about the condition of the magneto points, or the verification of the mag-engine timing. For the relatively low cost involved, you may wish to recheck the points condition and possibly replace them if there is a rough or frosty appearance to the contact finish.
I would not immediately suspect the spark plugs because if I interpret your message correctly, the engine seems smooth enough at operating RPM. Wire-electrode spark plugs perform best in VO engine installations, I have little experience with massive electrode plugs. Depending on the age and type of plugs, it is not impossible that a contribution from massive electrode plugs may result in your roughness. Additionally, I would tend to eliminate the engine harness because a fault within the magneto – spark plug wiring harness usually results in roughness at all RPM levels.
I hope you have the 1200 series Bendix-Scintilla magnetos which have always provided best service and reliability in the VO engine. A faulty magneto may not produce the desired spark intensity at low RPM while producing an apparently acceptable spark at high RPM. If the points and mag timing check out acceptable, I would suggest the magnetos as next investigation target. To verify the magnetos it may be necessary to remove them for bench test under load.