Hello.
Have 95 Pickup, Ka 24, 5spd. Runs great when cold. After warm up runs poorly at low RPM under load. Stumbles, hesitates. Seems most evident when accelerating from a dead stop. Idle is fine. Runs okay at high/full throttle. Anyone familiar with this problem?

I've had idle problems before, but not acceleration issues when hot. Do you have any misfire codes or a check engine light? If it only happens at low rpm when accelerating from an idle is the battery voltage dropping too much? Maybe there is a spark issue or injector issue caused by low voltage? It might also help to check for fuel pressure if the hesitation starts to occur anytime the engine is under high load.

Also, are you sure it's the engine and not the transmission?

Have no reason to think it's the tranny. It's a 5spd manual and the truck runs great when cold. I think it' likely vacuum related, but that could be a lot of territory.

If the engine stumbles under normal acceleration, in 2nd gear for example, but idles at 500-700 rpm, you might not have a vacuum leak. Vacuum leaks usually cause the engine to run lean at idle but normalize as the rpm increases because a higher percentage of the air entering the engine is measured by the Mass Airflow Sensor as intake air volume, and RPM, increase.
Does the truck idle faster than 700 rpm?
Do the symptoms decrease as the rpm's increase?
Do the symptoms occur in any gear, at any speed, at any time once the engine has warmed up but always below 2000 rpm?
Does it ever stumble during acceleration passing another vehicle on the highway?