My RPM keeps going up and down about 100 RPM for no reason. It seems to do it a lot more with the cruise on. Also it only seems to do it at RPM's below 2,000. Does it weather in over drive or 3rd gear. My tranny fliuid is super clean, so clean that maybe it's been rebuilt recently. There is no slipping when taking off from a stop or at any other time that I can notice. My guess is the torque converter. Anyone else had this problem? I just bought the truck a couple months ago and didn't notice it doing this until a week ago. The truck has 145,000 miles on it. Also, my speedometer is off a bit. It reads about 5 MPH too slow at 50 MPH. I bought it from a dealer, so I don't know any history other than the carfax was clean. Maybe the speedo was recalibrated by a previous owner who may have had bigger tires on it? Other than this, I really like the truck.