Shifting problem

Hello, I'm Danny.

As stated in this post you've attached to it sounds like you might be having a shift solenoid issue. I say this because you state the level is fine and the filter is new. The best way to check is see if there are any transmission codes stored in your trucks computer. Here is a tutorial showing how to test OBD1 GM codes:

https://www.2carpros.com/articles/buick-cadillac-chevy-gmc-oldsmobile-pontiac-gm-1983-1995-obd1-code-definitions-and-retrieval-method

If there is a transmission code stored get back to us with the code information so when can then help properly diagnose. Hope this helps and thanks for using 2CarPros.