When I drive on the highway everything is great, however when I drive on the county roads and it drives up the hills under a load, my truck seems to keep downshifting and upshifting at odd times. I did have a rebuilt trans installed last year and I have had no issues with it. Someone told me check all the vacuum lines I must have leakage.

Has anyone else had any issues like this? I am currently getting 12 mpg.

 

12 MPG on a v-10 is pretty good.....

Are you driving in overdrive? Right around 45 MPH??? If so, that's normal, trans is kicking in and out of O/D and probably TCC lock as well.

Try disabling O/D, and see if it behaves any different.

 

I did try disabling the O/D and it did not make a difference. This is something that just started happening in the last few weeks. I have had this new trans for over a year and everything shifted great.

 

Can you post a video of what its doing?

Might try checking the TV cable is adjusted properly, and not binding anywhere.....