driving the truck on highway ok around town when problem starts at 40 untill i hit 50 mph then truck ok the rpms will jump up down ANY IDEA 99 3500 WITH 5.9

 

First off, Welcome to DTR !!!

I moved your question to the proper forum where it will get a better response.

I believe you are hitting the lower limits for overdrive at that speed (like mnine does). The tranny is trying to do its job which is the help the engine run efficiently, but it is a machine (and a complicated one at that). From what you described it sounds normal. There really isn't a fix for it except to do what a lot of other people and myself do and push the O/D OFF button (overdrive lockout). This will prevent it from hunting and save the wear and tear on the O/D unit in the tranny. Hope this helps, and once again welcome to DTR.