1987 A4LD shifts back and forth between 3rd and 4th gear under acceleration

[whodunit]

My ranger has had this issue for a little while now. It shifts great until I'm giving it some gas (usually uphill) in 3rd and get above 35mph or so. When it shifts into 4th it will quickly drop back into 3rd and then go back into 4th and so forth until I either give less gas (so it stays in 4th) or give it more gas to basically keep it in 3rd. It feels like theres an area right between 3rd and 4th gear that the truck can't decide where it wants to be. Hopefully that makes sense so far, Its hard to explain.

I tested the vacuum modulator and any vacuum line I could get my hands on. The modulator held vacuum but the line to it didnt. I replaced the line and now it holds. I did find another leak or two in the process of searching the rest of the vacuum system and replaced all the tubes with new silicone ones but the shifting issue persists. I adjusted the transmission bands, cleaned the valve body and replaced the transmission filter and put fresh fluid in. I've replaced the TPS and the MAP sensor but I wasnt able to find motorcraft parts for those so I can't say for sure they're good parts.

The things I haven't done yet that are on my suspect list is inspect the servos, inspect the VSS and adjust the kick down cable. I'm wondering if anyone can help point me in the right direction based on the described symptoms.

Thank you!

 

[Fix4Dirt]

that could be normal, sometimes you are at an in between speed etc where it is always searching and hunting. did it not used to do that? just clearing that up

 

[Josh P]

What engine and what is the axle code on the door tag, assuming it is original.

 

[BrooklynBay]

Does it only happen when it's cold, hot or any time? The governor could be sticking but that's usually for 1-2 gear shifts. Is it possible that you feel the TCC cycling, and not the 3-4 shift?