A problem with the OD OFF wiring won't cause your problem. I think you have a problem with the relay that powers the computer. If the computer isn't powered it will do exactly what your trans is doing. I don't know where that relay is, but it is under the hood.
Have you tried reading the codes when it won't downshift? If I'm right you won't get any response when you try to get codes when it won't shift.
Have you tried reading the codes when it won't downshift? If I'm right you won't get any response when you try to get codes when it won't shift.