The gear is shifting 1 and 2 easily, then 3rd is hard and no 4th gear?

Greeting,
I have a 1991-1992 BMW 735i, 2weks ago the reverse stopped working, So i took it to the mechanic and told me that the oil is low so we added 2 liters and the reverse worked just fine, but there no speed changing from 3rd gear to 4th gear.
2 days later the problem went back again but it wan't low on oil, so he told me we need to change the transmission oil filter and we did and the car was ok until 4 days ago. same problem appeared again. The gear is shifting 1 and 2 easily, then 3rd is hard and no 4th gear.
Is the transmission is gone or it can be something we can fix.
Thank you so much for your help.

__________________
This post was auto-generated based upon a question asked on our tech article page here:

[Kerry at Pelican Parts]

Sounds like your valve body has sticking pistons. You can try a rebuilt valve body

- Kerry