2001 Dodge Ram torque convertor clutch engages and disengag

When I drive for 45 min to 1 hour, my torque convertor clutch seems to engage and disengage - I have rpm variance by 200 rpm at 75 mph after driving on the highway for a while. If I go slightly easier on the throttle or slightly heavier on the throttle, the torque convertor clutch stays engaged. It acts up more on bumps and it acts up less at lower speeds.