I had a similar issue when I fitted the Emerald M3D Ignition ECU. This retaines the distributor for switching using the lumention optical eye and spark distribution. However, the bobweights are secured shut as the ECU will take care of advance. The mistake I made was having the bob weights welded partially open, so the optical eye was itself partially advanced with respect to the rotor arm and cap position. Timing was set absolutely correct and everything worked as it should. After a while the engine faultered, popped where it shouldn't and failed to run smoothly. The 7 was flat bedded to Dave Walker at the old Emerald Cams site in SE London (back in the day, now moved). He found the rotor arm was firing at the correct cylinder but due to the advanced base plate was also partially cross firing to the next cylinder at the same time. This cross firing partially ignites a charge in the exhaust cycle causing the missfire situation. Dave filed a new slot in the distributor shaft to flip the rotor arm to a better position and moved the cap to suit (something like 180 degrees but i would need to look again). All has worked perfectly since. He called this cross firing. I only offer the story in case you have done something with your distributor sensor and this gives a clue. This will not show as an error in ignition timing value. King Willy