The easiest test for a plugged cat is with a vacuum gauge. Measure the vacuum at idle and then slowly increase engine speed to 2000rpms. The vacuum should be greater at 2000 unloaded rpms. If it drops then suspect a cat.
My first impression is a secondary ignition deficiency. Either running out of available voltage or finding a shorter path to ground as seconadry requirements go up. WAs that a Bosch rotor? If not or even if it was try the old one again.
Bosch Master, ASE Master, L1
33 years MB technician