I have an 86 F250 with a 4.9 six cylinder. The truck has 87000 miles and is in great shape. The truck idles poor. One minute it barely idles and the next it is revving at a couple thousand rpm. I have changed most of the vacum hoses (not all), new rotor cap, new plug wires and plugs, fuel filter, and air filter. What is next?

The truck is a workhorse but does not have a lot of power pulling a trailer with some lawnmowers on it. Any help is appreciated

Whitdog