I've been messing around with my 1989 W124 300E for about 6 weeks now, very similar symptoms. Today, I finally fixed the last piece of the problem.

The car seemed like the engine was constantly battling the transmission. It would run fine when cold (running richer), then sometimes at temperature, it would run really rough, downshift and slam into gear.
At WOT, the car would move, begrudgingly. In town, at low throttle, you were very likely to get missing, skipping, arguing car.

Yesterday I went to a UPull-it yard, and happened to find a recently wrecked car that someone had clearly just put a bunch of money into. I scored an OVP, a reman-fuel pump relay, and a fuel distributor all for $65.

The OVP and relay are spares ($10 each!!!!!)

The fuel distributor is what fixed my problem. It was even effecting how the car shifted. Night and day difference.

If you, your multimeter and fuel pressure gauges have you this far, it might be worth the $60 rebuild kit online, or a trip to the wrecking yard.