1998 Chevy Blazer

Ok this is about as close to the problem as I can get

1st -3rd fine. Works like a champ. Rev fine. Also works great. 3rd - 4th (OD) intermittent. When driving it will sit at 2500rpms at 60mph, you go to accelerate and it will drop down to where it should be if it were in over drive(about 1600rpms), but once you stop accelerating and maintain speed it will go back to the 2500rpms. Does the same thing with cruise control on as well. Its not the TPS that was just changed 3 days ago. It doesn't shift hard, and seems to run fine at the 2500rpms, just its not gas efficient. Please help if possible