I've had this issue lately where I'll be cruising along between 55-65 mph in cruise control, and every now and then it will sound like the transmission slips for a fraction of a second. I'm assuming it's the cruise control momentarily adjusting, but I have no idea what could be causing this. It's almost like the engine revs for a moment then goes back to where it was. It's not the transmission slipping, as it never happens with cruise control off. Any ideas what could be causing this?

 

I would suspect it happens without cruise as well, you just don't notice it as much. Try this. Find a nice stretch of road, where you can maintain 55 mph. (with your foot on the gas. not cruise control) Once you have reached target speed, pay attention to the tach, (and the road.... maybe bring a friend along... to watch the tach) and vary the pressure on the gas pedal just a little bit. Does the tach bounce about 200 RPM or so?

 

It does not. Every day I drive to work, there is a good 5 minute stretch of road that is 65. Some days I hold it manually, some days I use cruise. It only happens in cruise. I did this exact experiment you described to determine when it was happening, and determined it was only with CC on.

 

Well, that makes life far more interesting. How much of an RPM change do you see? If it's only 100-200 RPM, it's likely it just the TCC unlocking/locking. Normally, I would point at the the TPS for that symptom, but, if that were the case, it would do it whether you were on cruise control or not. Since it doesn't when not using cruise.......

What year is your truck? Engine? Trans? And anything else you may deem pertinent. (like, and work on it recently? How long has it been doing it? Gotten any better/worse with changing conditions?)