Gearbox behaviour is modified by lots of things, including how much throttle you have in. There will be a connection from the throttle to the box by a cable somewhere. The kickdown switch will come in when you press MORE after reaching WOT, to tell the box you want to go down a gear for some more pick up. If you werent pressing the throttle right to the boards, it wont be relevant to your problem. You should find 3 connections to throttle at engine - actual throttle, moved by the pedal, cruise control cable or similar, and another going down to the box. Sounds like the gearbox connection is loose or broken or jammed in its casing. The gearbox is also supplied by a vacuum connection (dont know your model, maybe wrong here), which if broken will give hard shifts as though you are WOT, and a poor idle on the side.
HTH
jp