E300 M103 KE-Jetronic lack of power when warm
 

I’m having issues with my E300 1991 M103. Runs great at cold start but the warmer it gets the less I throttle I can give without choking/stalling it. It really feels like it's running too lean, more starvation the warmer it gets.

If I continue driving it (lower and lower speed to avoid stalling it with too much throttle) until full operating temperature it eventually starts to run poorly even on idle. If I then turn it off and restart it immediately, it backfires some and takes 5-10s before it starts and runs poorly.

If I wait until it's cooled down it runs great and accepts full throttle again.

Has anyone else experienced this problem?

That behavior is outside the range of water/air temperature adjustments, so those sensors are not immediately suspect.

Check for condition of:
O2 sensor
Fuel pumps (one or both may be failing as run time/temperature increase)

I also forgot to mention that if I push down the fuel distributor sensor plate a little by hand while giving full throttle at same time it seems more powerful than ever (when its warm).

Both fuel pumps at least sounds like they are working, also when its hot.

O2 sensor I haven't checked. Is it affecting the mixture also on absolute full throttle?

Between the fuel distributor and the air flow sensor there is a small tower with a 3mm socket screw inside. That is the main mixture adjustment.
A hex wrench can be inserted, and the screw pushed down against a spring. When the screw is slowly turned it can be felt to go down a little farther, and to engage a second screw. The second screw is the actual adjustment. Try 1/4 of a turn Clockwise (CW). There should be a noticeable change if that adjustment is the culprit. If there is an improvement proceed CW in increments of 1/8 turn.

if the top of the tower is cut off, that is.

Quite true; it would be startling to find one that has not been opened, however!

EHA can cause it

Check the boot under the fuel distributor and the hoses going to it for cracks.