My '98 K1500 Chevy will change rpms while going down the road at highways speeds. It seems like the torque converter is locking and unlocking. It changes just a few hundred rpms. I first noticed it when I would let off the throttle partly. Now it seems to do it more often. It shifts through the gears fine and when I get 70+ there is no problem. I am thinking it is the transmission ( supposedly there is a spring and check ball that is responsible for the torque converter locking up) but I am reading about other causes like a dirty maf sensor or a failing TPS sensor. Anyone had this problem before? She is my hay hauler and feed getter so she can't go down. Appreciate the help.