If it has an Engine Coolant Temp Sensor (ECT), two wires, then it may not be changing it resistance as the engine warms. In that case the computer will still be in cold start rich mode when warm - and it will idle poorly, but run better at speed.

You can induce a small vacuum leak at idle to see if the idle RPM picks up. This will help to induce more air into a rich mixture and balance A/F ratio.

92 Montero LS 3.0L V6 Auto, Stock, Original owner, 185,800K miles