Been having a similar problem with my Vitesse fitted with lumination electronic ignition, started to misfire after it warmed up. Ended up disconnecting the electronic ignition and fitted a spare distributer complete with ancient leads, cap, points, condenser and rotor arm with rivet. No sign of misfire since, will have to try the cap, leads and red rotor arm from other distributer to isolate the problem at some point.
Regards
Paul