The clutch packs in these beasts just don't wear, it's usually the governor, pump or varnish in the valve body. Seeing that you can manually shift it successfully it leads to those three if in fact it is a tranny issue. I think not.

 

Okay, good to know about the clutch packs. So, in some cases, someone may have been having problems with the transmission before I bought it and replaced the trans oil. I cannot confirm that the filter or screen was replaced - correct me if I'm wrong, I thought this transmission only had a screen.

If it in indeed the governor or the valve body, I've found those can be done with the transmission in the car. The pump however, that leads into a far more costly and time-consuming venture - ie: transmission R&R or replacement.

I've searched around a bit for information on swapping out the governor and valve-body, but haven't found a lot of details. Could anyone elaborate?

Thanks again all - slowly deciphering the problem!