Hello Franko

New wild guesses!!!
#1. Rack dampener pin is bad.
#2. Bad valve adjustment.
#3. EGR jammed open by carbon/oil sludge.
#4. Air leak in fuel feed lines = cracked hoses or lines.
#8. ALDA needs to be tweaked.

Good. RE: Changed or cleaned all the filters including fuel tank screen.

An O ring, missing aluminum seal washer or loose secondary fuel filter will fit your symptoms on start-up; also a failed glow plug system will cause this issue, and bad valve adjustment will contribute to this. RE: Just by the feel of it, (needing to crank a few times even when hot, after being off for 5 minutes or so, I got the feeling that there might be an air leak in the supply, somewhere along the line.

Normal. RE: Is there supposed to be an air bubble in the clear in-line fuel filter?

He used calibrated ears???
Do your own valve lash check , and consider a new mechanic, one misadjusted intake valve can do strange things , ask “1985 300SD Sady” how easy it is to check the valves, he is 15 years old . RE: The Master in Sav said that the valves were not out of adjustment, because it has great acceleration and power. ( I had posed the possibility to him)

Good. RE: I’ve tested the bad tank vent theory by removing the fuel tank cap, with no result.

No. RE: Should I be checking the vacuum system some other way?

Remove the silver tube clamp, cut a soda can, use at least two layers between pipe and valve, reinstall clamp and test drive. RE: How would I check the ERG?
Yes. RE: Is it the semi-shiny silver round thing near and in front of the air cleaner?

Last resort IMO, but still possible cause. RE: You mention the ALDA, but if it were out of adjustment, wouldn't that diminish power?

I would need to drive to confirm. RE: This car has great power and top end.

There are many threads on this subject, and it is DIY simple. RE: This leaves the rack dampener pin, of which I know absolutely nothing.

No relation to silly jokes. RE: Sounds like the bacon-stretcher I was sent to get, back a few decades, in the Boy Scouts.

This sounds like glow plug system failure. RE: Another hint might be that even in 45 degree temps, this thing hates starting. SO unlike my old 300sd, from so long ago.

Atmospheric pressure and temperature are different. RE: Why does it run so well in Savannah?

Where are you in Ohio, PM me, there may be local help. RE: I don’t get it. It still wants a little help to start there, but it runs so well. I mean, it's like silk there....smooth even idle, like a metronome, perfect as can be, and then back in Ohio; bump bang choke miss..

This sounds like a classic rack dampener gone bad. RE: It tore out the motor mounts here, it was so bad.