It is most likely either the resistor or the connector going to the resistor. Sometimes those connectors get corroded and make marginal contact. Before replacing the resistor, take off the connector, clean it as well as you can with a little WD40 and an old toothbrush, dry it off with conpressed air, and slide it on and off several times. Then see if it works OK. If not, then change the resistor.

1977 CJ-7, fiberglass body, AMC 360 w/ headers, DUI ignition, Edelbrock intake and Holley 4150 carb, TF999, Dana 300, 4.56 gears lockers, York air comp, 4" susp lift, 2" body lift, BFG 35" M-T tires, Megashifter, AGR pump & box, REP8000 winch.