Daughter had a 2005 Ford Exporer (4.0 V6 with R55W transmission) and when it got below the minimum cruise speed, the engine would quit. Apparently the Throttle Plate Idle Position is electronically controlled by PCM/Cruise and Cruise at low speed confused that.
Oh, If you have a Ford with the OHC heads on a 4.0 V6, be warned about that engine's longevity. One camshaft drive chain is at the REAR of the engine. Engine, maybe Transmission, has to come out to replace that "pack" of Chain, Tensioners, etc. Job will cost more than the value of the vehicle. Anything north of 100,000 would make me nervous. Explorer, Sport Trac, Ranger... The 4.0 push rod engine is OK.