I had the same problem with my 1987 Rangie I fixed it by disconnecting the thermo time switch, the engine runs a lot more smoothly at all temperatures and speeds but takes a couple of seconds longer to start. if cold starting is an issue replace the thermo time switch. also check the cold start...

I had the same problem and fixed it by disconnecting the thermo time switch, it means the engine takes a slightly longer time to start from cold but now runs smoothly and more economically