* Low transmission fluid: Check the transmission fluid level and add more if necessary.
* Dirty transmission fluid: Change the transmission fluid and filter.
* Faulty shift solenoids: The shift solenoids control the flow of transmission fluid to the different gears. If a shift solenoid is faulty, it can cause the transmission to shift back and forth between gears.
* Faulty transmission control module (TCM): The TCM is the computer that controls the transmission. If the TCM is faulty, it can send incorrect signals to the shift solenoids, causing the transmission to shift back and forth between gears.
If you are not comfortable troubleshooting and repairing your own transmission, it is best to take your vehicle to a qualified mechanic for diagnosis and repair.