The T400 doesnt use a downshift cable, so it can't be that. Check the vacuum modulator at tranny. It has a vac line that goes to manifold vacuum and controls upshift. See if line is connected, and the modulator isnt leaking tranny fluid. It'll do that if the modulator diaphram is shot. TH400s use a downshift switch, check switch and wiring.
You can pull pan and check for metal or crapola in pan. Could be a sticking valve in valvebody, worn clutch packs, plugged filter, wrong fluid, band adjustment, shifter adjustment, govenor sticking, or bad torque converter. Hear any buzzing, whining or other noises? Find any serious metal in the pan, and its curtains for that build....
good luck
chip
