95 E320 rough idle and running - transmission related?
 

Have new plugs and the wiring harness looks fine so either it was replaced or is new enough.

Engine starts perfectly but idle will drop for a second several times per minute. Sometimes less, sometimes more.

When car is in gear and this happens the engine will shudder a bit as if a load has been put on it (transmission?). In neutral you can barely notice the idle drop.

When driving and accelerating strongly or even up a hill the engine runs quite roughly and there isn't much power.

Problem is intermittent but getting worse. There are certainly times where I can floor it at 30 and fly effortlessly onto the freeway. Othertimes the car struggles to go up a hill at 25 mph.

New variation is a snap or popping sound from the rear of the engine or front of the transmission.

I think I might have to take this one to the shop...

It sounds like ignition misfire due to bad spark plug connectors under one of the 3 ignition coils. Techs usually replace these on principle when they replace plugs (you can search this site for more). Have you tried to retrieve any diagnostic codes? Mark