Fast idle on F-150

I have a 1993 F-150 4X4 5-speed manual transmission 302 engine with 170K miles on it. Often when driving, the idle speed will increase to as much as 3000 RPM between shifts. Sometimes I can go a couple of weeks with no problem. Sometimes it has the problem almost continuously. Temperature does not seem to affect the problem. The truck and drive train are in excellent condition and nothing has ever been worked on since new.