shifting from 3rd to 4th continously
 

For the first 15 to 20 minutes of driving my transmission shifet from 3rd to 4th continously. Back and forth like it's confused what gear it should be in. It accelerates fine and doesnt really have a problem until I level out at my cruising speed. 45 - 60. After about 20 minutes it will calm down and stay in 4th for the most part. But it rarely ever goes into overdrive and if it does it's only for a brief moment. And would this problem also contribute to slow shifting especially when getting into the throttle. Can take up to almost 2 seconds and if I'm in the throttle it will rev up real high in between gears. Do I need to replace my tranny or is there a easier cheaper way