Sometimes the the motor will be turning 2100rpm doing say 70mph and then the after 5 miles down the road or just a mile down the road(just depends on when it wants to do it) it will jump up to 2300rpm in order to run the 70mph i was just running...im thinking its the torque converter locking in and out, but im not sure...what do yall think?...and how to fix this problem?

it was doing this when the 4.3 was bolted to it and still doing it with the 5.3