re: Most of those golden oldies don't ever come to dealers

Yep, a water pump on a W124 will make you think that way.

For those interested, here is the misfire monitor method from the Bosch manual. Keep in mind that the ECU is juggling a lot of things at one time...

 

You very well could have a sticky valve caused by engine sludge then.
5w-30 is the wrong oil for these engines.
5w-40 or 15w-40 diesel oil only, anything else other than a true full synthetic and you will have problems.
Start with the oil change, the diesel oil will help clean up and engine sludge and free up a sticky valve.

 

Or run the engine with 5 qts of oil and a qt of trans fluid for 500 miles. Then refill with proper oil.

 

if it is intermittent, then do two things, add coolant dye and check for coolant in the oil or the spark plug and or cylinder. secondly, use some bg 44k and use it regularly. that might help cleaning the exhaust valves. the injection service can kinda work but it does mostly the intake valves. the exhaust valves are the problem child on those motors.