What would cause a rough idle when cold and at idle but runs great once warm

Sounds like a vacuum leak

Error Code: P0221
Description:
Throttle Position (TP) Switch/Sensor B
Accelerator Pedal Position (APP) Switch/Sensor B Range/Performance Problem
Possible Cause:
- Accelerator cable adjustment.
- Throttle position (TP) switch/sensor .
- Accelerator pedal position (APP) switch/sensor.
- Check valve clearance, adjust if required.

Leaking intake manifold (vacuum )

Replace the temperature sending unit/sensor. This sensor tells the computer to put the engine into cold start mode and it then sets idle a bit higher and puts a bit more fuel into the engine to keep it running without having to touch the throttle. Once it's warm, sensor tells computer to go to normal run mode and it's done. No problem.

1988 ranger 2.9 runs great cold then after a few minutes when warmed up hesitates with intermittent black smoke from the tail pipe until i hit the the throttle then clears and will repeat this behavior...

Have you had your mass air meter checked or replaced? This is the unit attached to the air intake hose behind your air filter. It senses the air temperature, speed, density, etc... and adjusts the fuel/air ratio to keep the vehicle running at "peak" performance. Also, check your TPS (throttle positioning sensor) it can also cause the problem you speak of. The TPS adjusts your engine speed/idle etc. and also compensates for hot/cold changes etc. This is roughly a $35.00 part and in most cases usually an easy fix.

check for minor vacuum leak and map sensor,pcv,egr valve

try coolant temperature sensor

try the throttle position sensor

Have a mechanic take a look at your throttle position sensor and engine idle sensor. Make him manually adjust them as opposed to just putting in new ones. It may require that you have to have a higher idle than normal. Your underlying cause is going to be that I am sure you have a lot of miles on your engine.