Ok .... Off on a complete tangent here, but a couple of things worth checking.
1. Are the intake tubes nice and tight? The rubbers each end can become hard and if the jubilee clips aren't that tight they can become loose. This can cause a lean mix on one pot, which can lead to plug or worse valve failure. Usually this will not cause a mag drop but shows as a slightly rough running and especially in the mid revs range. However, this can appear worse as one plug is switched off. Probably not the cause, but it's a simple check that's worth doing and is often overlooked.
2. Mag timing, remembering that the left mag is usually a couple of degrees retarded over the right, is it possible the timing on the right mag may have moved slightly too advanced? Are the nuts good and tight? Are the mags secured using spring washers, or stiffnuts and not nylocks ... the latter can theoretically loose their grip due to heat cycles and shouldn't be used on engine componants really. Being a PFA aircraft, never dismiss poor aircraft practices from previous owners.
Left mag knackered - or maybe just damp.
Right mag is the one with the bigger drop?
What would cause this? I didn't know the fuel/air mixture got inside the magnetos
I think Flik must have meant plugs?
All other advise re damp, knackered leads ect very valid. It could of course simply be a case of knackered mag though
SS