1990 Mazda 626 Car runs good for 20 minutes then trany fail

My Mazda 626 DX will start up good and we can drive it for about 17 minutes before gears stop working. Reverse, Low and second go out then after 20 minutes of driving drive stops working and the car will not move anymore. (The car will continue to run after all the gears stop working) If I let the car sit and cool off it will move again but only for 20 minutes maximum. All the fluids are good, all filters are good not sure why the gears stop working when the car warms up. It gives no warning signs before the gears go out. Do I need to rebuild the transmission or is it as simple as getting new seals?