280E Runs Great! then Runs Bad!!

[77280E]

I’m new to the world of vintage Mercedes and new to this message board. I'm enjoying the car and really appreciating the quality of the engineering and workmanship. It's slow at the bottom end, but amazing at the top end.

Here's the problem: 1977 280E - It starts fine when cold and runs well. 15 ~ 20 minutes into the drive, it starts to run bad, idles rough, and stalls at stops. I can park the car for just a few minutes, and then it purrs at idles and runs well again.

* I've installed new plugs, new cap & rotor, new K&N air filter, run injector cleaner through the gas tank once, checked for vacuum leaks.

Any ideas? Thank you in advance for your tips, knowledge, and insights.