I would plug in the Crown Service Terminal into the transistor controller (there should be a port under the operator compartment cover). This will allow you to pull codes and edit any speeds or other settings. There really isn't any other way to test the transistor besides just replacing the current one with a new one to see if that fixes your issue.
Otherwise you can test the drive motor and make sure it is getting the correct voltage and not testing bad.
From you symptoms I do belive it has something to do with the transistor controller itself or how it was programmed.
Did this lift ever run faster or has it always been slow?