92 Paseo whacking out

92 Toyota Paseo AT- my sister's car, always starts and runs great. Until. When she is going down a long hill about 5-miles from home the temp guage drops and pegs out cold and by the time she gets to bottom of hill the tranny downshifts and stays there for 3-4 miles (revving engine the whole way unless she really slows down) then temp guage goes back up and tranny upshifts and all is fine? It has done this the same way in same place for months, and it seems to happen on cold or cooler days, rarely on warm days. Temp sensor?