Slight surging between 65 & 70 mph - overdrive shifting in and out?

[Sunbelt56]

I get up to 65mph on the highway and try to slightly accelerate and between 65 & 70 mph I feel the truck surging like it's going in and out of overdrive. The transmission fluid was recently changed and it did the same thing before. It seems to be fine when it gets over 70mph. I can get it up to 80mph with no problems. Oh, by the way: it is a 1994 Chevy 1 ton p.u. w/ a 7.4L automatic and the gear ratio is 4.56. I'm getting around 10mpg highway. Recent tuneup and oil-change.

 

[PlayingWithTBI]

A lot of guys have had similar issues and it turned out to be the ICM (Ignition Control Module) inside the distributor. Mine was surging during cruising at ~2000RPM in 4th gear. I replaced the nearly brand new one from Spectra Premium with an AC Delco plus Arctic MX-4 thermal compound. It's been running smooth for months now.

 

[Sunbelt56]

I found a broken vacuum line going to the sensor (IAC?) next to the thermostat housing. I think that fixed it.