My 93 420SEL started acting funning the other day . At first I thought it was a bad shift between the 2nd and 3Rd gear but thats just the RPM range that I first notice the problem

The engine hesitated and almost stalled around 1700 RPM under fairly aggressive acceleration ( heavy load). I sorta babied the throttle, and backed off when I first felt it , thinking it was the tranny. It happened a few more time when I test drove it around trying to figure out if it was engine or tranny .
I was able to drive back 30 miles home at highway speeds and nothing seemed out of the ordinary. It seemed to happen only when pulling away aggressively from a stop.

Engine idled smoothly but quit once after starting it up and being very cold here in NYC

I brought it to my 2 partner mechanics who worked 15 years for Benz dealership and the one mechanic reved the engine in drive (1600 RPM to load it ) and replicated the breaking up. He says it breaks up under load prolly because of the coils. His partner says to replace the rotors and caps. Two partners cant agree and want to start changing parts without even putting a diagnostic tool on the engine? I ran away !

I brought it to the Dealer and the 25+ year mechanic said it gave him no codes but he replaced 3 resisitors that go between wires and plugs and the 8 plugs, scraped the rotor caps with a wire brush and told me put a can of techron in a full tank . His suggestion was if it continues to hesitate, to change plug wires and then possiblely injectors.

Drove 350 mixed miles over last 9 days and in the last 4 days I can usually go all day with 3-4 engine turn offs and restarts and maybe one HICCUP/FLAT SPOT / hesitation per day.

Once past 3000 RPM it pulls like a beast on the highway . So I thought the trend was going in the right direction and dropped by my dealer mechanic to say so, give him a tip etc . and ask if I should try another techron in a full tank of gas and he said ok., and to not change any parts and see if it continues to get better.

No sooner did I leave the dealer and it had that same hiccup . Also after buying the techron and not having a chance to put it in, it stumbled once in 10 mins driving . Then I turned it off for 40 minutes to get something to eat and when I started it up it sorta stubled on idle, and once accelerating aggressively, and then again accelerating aggressively, but this time about about 2800 RPM.

So much for the trend. It never happens if I feather the throttle and when I back off the throttle when its doing its hiccup, it recovers and I feather thru that spot and it drives fine. I have had it running at 60-80 MPH smoothly for more than 300 miles .

He said I can put a resistor on a bench and it tests perfect then put it in a car and it does not perform correctly. What scares me is that even he was not really certain what caused it, and told me to not change anything yet, but as soon as I left his shop it got a little worse.

Sorry for my being so lengthy, any ideas ?