based on what you see by disconnecting the cable. Did you check that the cable is seated correctly at the tranny end?

If the cable is OK, it sounds like you need more RPM than expected to generate correct control pressure. That could be the pump or valve. Given that it's isolated to the 3/4 shift, that would point to a sticking valve in the valve body.

Note: I am not a transmission expert. Just a former 201 owner who likes to think out loud.