W123 240D hesitation/loss of power on highway Why?

[herring]

I got my car back from the mechanic a few weeks ago and on Sunday I took it out for its first highway trip. Engine and transmission were well warmed-up from in-town driving before I hit the highway. After a few minutes at full speed (65-70 mph), I noticed a sort of intermittent "drag", like some force was trying to slow the car down. It would come and go in short spurts.

The highway ended soon. On the way home, I decided to take a different route, with more highway. The car drove perfectly normally on country roads (35-50 mph) for half an hour. Once on the highway, the hesitation/drag/whatever reappeared after a minute or two. It worsened to where the car would not keep up over 60 mph, but didn't continue getting worse. I put the car in neutral and revved the engine through various speeds...everything seemed fine...no unusual roughness or sounds.

We got into the city and the car drove normally in stop-and-go. Once out on the highway, it took a minute or two and the drag effect reappeared.

After getting home, I let the car idle while unloading it and telling my wife about the trip (10-15 minutes). I then checked the transmission fluid. It was a nice pink color and the level was between the marks. I had replaced it myself in the spring, and thought I might have overfilled it a bit but with the well warmed, then cooled, transmission it looks to be ok.

I am at a complete loss on what this could be. It really does not seem to be an engine issue, I'm thinking either transmission or some other drivetrain problem. Can a failing bearing act like this? The fact that it's taking a couple minutes to start makes me think something has to heat up for the problem to present, and in-town driving doesn't make that happen. Anything I can easily test for? My time is limited these days because of family stuff.

I can bring it back to the mechanic but would like to have some idea of what I'm dealing with. He commented that the car drives really well (has power) for a 240D automatic of its mileage. This is an original engine and transmission. Odometer died at 307,000 miles, my best guess is it's about 325,000 miles now.