While hauling our TT on highway at 65 MPH (within weight limits), the transmission downshifted to 3rd gear and would not come out. The wrench light came on dashboard. When I shut down Navigator, wrench light went off and then everything returned to normal. Transmission shop rebuilt tranny, installed new torque converter and heavy duty discs. Also put in new alternator and condenser. After rebuild, the problem returned and the mechanic road tested and could not find a problem. Mechanic cleared all codes and test drove with TT with no problems. Next time out, exact problem came back. Took it back to transmission shop and they installed a new torque converter and replaced a suspected valve. Test drove with TT problem returned, wrench light on dash and downshift. Same solution. Pull off; shut sown; everything returns to normal. At a loss. Any Ideas will be greatly appreciated.