1998 Dodge ram autotrans problem

I have a 98 Dodge Ram 1500 4x4 with an auto trans. Sometimes when I am in overdrive, I notice that the RPM's will fluctuate about 300-500 rpms back and forth (like the RPM's gauge is waiving at me). It only happens in overdrive and not all the time. Any ideas? How long has it been since it's had a tranny fluid and filter change?

It sounds like your torque converter clutch is trying to lock up (like it's supposed to), but can't quite lock up.

This transmission should be able to be scanned for codes, but it takes specialized equipment usually only found at transmission shops or the dealership. I has been a while since it's been changed. I'll get it serviced and see if that helps. Thanks for the feedback. You need to take your truck to a dodge dealer. There is a TSB to reprogam the TCM for this condition. Changing fluids will not help this condition.