Hello

When I start my car cold and set it in OD and driving out on the highway the transmission starts around 54mph shifting up and down a few minuts. After a few minuts it`s stops shifing up and down. And the rpm shink to 1600 in OD. If i set it in Drive when it doing this, the rpm holds stedy on 2500. I hope you understand it.

 

I'm going to guess that it is the converter locking and unlocking. I would start by checking the fluid level. Either that or make sure your throttle position sensor is working correctly. That is one of the sensors used for transmission shifting.

 

Pretty common thing on these trucks. Shift solenoid(s) can be part of the problem.

 

The 4L60/e is notoious for having this problem.Other than shifting cylinoids causing it also it can be the boost valve that controls line presure or the proper presure of the fluid for shifting.Its recogmend to be changed at 75k.

 

shift censor sounds likly but the B censor which control 2to shift to 3rd and 4th.most common problem on 4l60e is censors shift and boost valve failure. changing censors change boost valve same time only have to drop pan and filter. however it may not be tranny related at all it may be the tampon in fuel tank attatched to bottom of fuel pump clogged up had many cars do this and turned out clogged up pump strainer.makes jerking shifts back and forth if no jerking then go back to tranny censors and boost valve. but if reverse works good goes right in and grabs good boost valve not bad its main purpose is for reverse which needs the most amount of pressure more than likly shift censors theyre 18 dollars a piece at advcance auto hange both