i got a 99 5.3 with 4l60 cammed and stalled.
when driving at highway speeds(65 - 75) my truck starts shifting in and out of over drive. any one have any idea what it could be?

 

I'd guess at the TCC is acting up.

 

thats what my uncle was thinking it was but i thought i would get a couple other opinions before i do anything about it.

 

Is it shifting in/out of OD, or is the TCC locking and unlocking?

 

i guess im not really sure.
but when driving 75 im turning like 1900 rpm normally. but when it starts acting up it, it keeps going from 1900 to 2500 just back and forth. but its not constantly its like ill drive for a couple minutes at 1900 then it will switch down to 2500 for 10 seconds or so and back to 1900 for a couple minutes.