1992 rodeo 2.6 intermittent rough idle
 

Hi, I have a 1992 isuzu rodeo I hope someone can give me advice on. At cold start, it fires right up and idles smooth. If allowed to idle, after a few minutes, it's like the idle drops very low, it barely runs and is very rough. As soon as it reaches operating temp, it smooths out and runs great. After warmed up and it sits about 20 minutes or so, as soon as you start it, it runs rough until it warms up again. While it's running rough, you can give it just a little throttle and the idle picks up and is smooth. I've tried setting the idle up so it runs smooth in its warming up phase, but then the idle speed is to high when it warms up. Also, if started cold and driven, it will sometimes run smooth at stops and will sometimes have a "surging" idle.
I am at a loss, I'm not sure how the computer controls the idle, it's not equipped with an idle control motor. There are no vacuum leaks, the spark plugs, cap, rotor, and wires are okay. As I said, I am at a loss. I hope someone has run into this before. Aside from the idle, it runs great.
Thanks in advance,
Ken