Big-time Mag Drop

912ULS in RV12  1100 hrs TT.     Recently replaced ignition unit with new.  Other one has 100 hrs on it.  Mag checks have been excellent since new unit and Roger's plug wire clipping procedure 2 hours ago.  New plugs all around.  I check ignition units before and after flights.  Last week all was well.  Today I got a huge drop of 3000 RPM on Unit B.  Very rough, wants to die.  Checked several times before going back to the hangar.  Very severe mag drop.  Not one or two plugs.  Other unit check was good. The condition apparently developed sometime after a very good post-flight check last week and today.  Plane in closed, locked hangar, weather has been mild in 40-60f . No significant moisture.   I did not have time to check which unit was at fault but wanted to get some expert guidance before going back to the hangar.  I would especially appreciate specific procedures applicable after I find fault with A or B side.  I'm hoping someone can point me in the right direction.   TIA    Jim in South Texas