Transmission not shifting right?

1987 Nissan Maxima mileage: 95,000. When overdrive button is on the vehicle will shift in to a locked converter mode after appropriate speed. The problem while driving on interstates that have long down grades is my engine temperature falls, and when you nearly get at the bottom the transmission shifts out of overdrive and causes the vehicles rpms to surge ahead about 700 rpm like you down shifted (actually slows vehicle like you down shifted) and it will not return to normal until you have driven sufficiently to recover some of the engine temperature, the overdrive feature starts working again. Transmission was completely flushed and new oil put in by a machine at a under car facility. Oil is still nice and pink, seems to smell okay. Really ran smooth in the warmer weather. Temperature now that is giving me these troubles is about 28/29 degrees outside.

The overdrive in your transmission is controlled by a temperature sensor on the engine. If the engine temperature gets too cold, it will drop it out of overdrive. Try replacing the thermostat in your car with a 195 degree one.

Thanks very much for the guidance it is much appreciated. This has been an on going problem for the last couple of years and as I may have mentioned, 3 mechanics and the dealer could not fix the problem. I will double check all the connections.

Transmission problem
1990 Nissan Maxima 6 cyl Two Wheel Drive Automatic

the transmission keep downshifting when the car get up to about 50 miles per hour, it slow down to about 25 miles per hour and wont go any faster.