1997 Nissan Truck will not idle below 1200 rpm

The truck runs fine once you got above 1000+ rpm. If you let it idle normal it shakes and dies. On the road it is fine. I had it in a local shop, they ran tests for therr hours, and found nothing. Everything checked out. The chekk engine light is on and it indicated that it was running rich, that was all they could find out. ( I bought the truck new )
Thanks