I recntly bought a 91 W250, 5.2 with toilet bowl injection and 435 manual tranny. Has a flat spot on acceleration tip in and on hills, I noticed pulling my boat that RPM's have to be kept quite high under heavier load or it hesitates/misses almost like cuting out untill higher RPM's are reached. No typical 318 lugging allowed here, not even a little bit. WOT seems just fine. Running empty I don't notice this so much except on moderate to rapid throttle tip in. Once the RPM's pick up it's fine empty or loaded.

Been all through the fuel flow and pressure, tune up, etc, common stuff, cat is clear, has a straight thru muffler.

Along the way I found the idle speed control is AWOL, as in not there. Is the missing idle speed control a likely cause of tip in hesitation and moderate load throttle response?

 

Idle speed control has no function once the throttle gets past its control area near idle. I would look at the TPS first as it can develop dead spots that can create stumble or loss of power at certain throttle positions. These symptoms would be accentuated under load.