1988 Dodge Ram (2x4) with pulsing acceleration idle

I have a 1988 Dodge Ram 2x4 with about 130,000 miles. It looks great and has been a great truck for a long time. I pretty much only drive it on the weekends (I’m a weekday bike commuter). For the last 2 or 3 months the idle has been pulsing with acceleration. Even as I’m in gear, if my foot is off the gas I can feel the truck accelerate for 2 or 3 seconds and then deccelerate. It’s very odd. I took it to my regular mechanics (who are friendly and honest, but don’t seem to get too worked up if something’s not perfect on my old truck). They told me they could hear the problem, but that they couldn’t diagnose it with their computer – “no codes” or the like. They said they don’t like to try to fix something if they don’t know what’s wrong. Which makes sense – to a point. Any way, it is a problem and it seems like a mechanic might be able to figure it out – fuel pump? Any suggestions would be greatly appreciated.