[V8] rough idle and when engine has reached proper running temp.

[Chris Brown]

 Hello.  I have q 1990 v8 4.2.
  Recently put the motor in and its a good motor. When i start the car when the engine is cold it idles perfect and is very responsive with the accelerator. Once i drive it for about 20 minutes and the car has warmed up, it is the opposite. Idling a little too low and when i press down on gas pedal she hesitates a bit and there is a brief second it almost feels like a miss. Once i get the car up to about 30 mph it is fine and runs perfect. Only happens after car warms up and at idle and leaving stop signs and such.  
What does this thing have for a choke? 
I was thinking air filter or fuel filter or oxygen sensor but it operates perfect when cold so im stumped?
     thanks 
  Chris
   1990 v8 4.2