Depends what It's doing really. Most times the trans is in limp mode when I buy them. I check the fluid condition on the stick 1st. If it's bad I change it immediately before trying any further diagnosis. If the fluid looks decent I go through connections and check for any codes, if I find a loose connection or pull a code, I fix the issue, then road test again. If that doesn't solve it, I change the pack.
Only once have I had 1 or all of those not solve the issue.

 

Awesome. I always heard that a transmission is very hardy. Think about all the abuse that they take, and they still keep going. I heard, as you found out, that it is much more likely to be an issue like you described rather than actually needing a rebuild or a new transmission.

Knowing this is a great way to get many a good car cheap. People see their car not shifting right and just give up, or they get a wrong diagnosis and give up.