1994 Toyota Pickup will not heat up

New radiator, thermostate, hoses, timing belt, waterpump was done when the weather was warm. The truck seemed to be fine but not driven much. When the weather got colder(30s and 40s) The temperature gauge only comes up 3/4 or so and the idel stays up around 1400 to 1700. Gas millage is poor. If you set in trafic it will nearly come up to temperature but then drops when your moving. It was thought the thermostate was in backwards or bad, so that was checked and replaced. Is there anything else I could check. My buddy has a SNAP ON computer so I had him plug it in but there still wasnt anything that jumped out at us wrong.