This happens 10 percent of the time. Can make car shift by stopping the cad and starting over or driving the car at a steady speed for about 300 yards.
Automatic transmission. Won't shift from first to second at times. on 1993 Nissan 240SX
Problem is most likely in the transmission itself. Servicing the transmission may be the first and cheapest place to start. In servicing the transmission the condition of the oil will be checked, the oil can be checked for debris, the filter and magnets at the bottom of the oil pan can be inspected, and on a test drive afterward the shifting of the transmission can be checked to see if the transmission may have further mechanical or electrical problems. Try adding Lubegaurd to the transmission oil after it has been changed it is a good product.
It is a bad speed sensor in the transmission. You can replace it or disconnect it at the fuse box in the engine. Happened to mine, after tons of searching on the internet, disconnecting it was the answer. As long as the speedometer works, it is the secondary unit for the computer to get the speed of the transmission.