Possible reasons:
Mechanical problems:
* Worn or dirty clutches
*Low transmission fluid level
* Damage to the torque converter
* Belt or chain slippage
* Faulty valves or solenoids
Electrical problems:
* Faulty speed sensors
*Transmission control module (TCM) errors
*Wiring problems
Other problems:
* Dirty transmission fluid filter
* Incorrectly adjusted gear shift cable
* Worn or damaged engine or transmission mounts
Solution:
1. Check the level and condition of the transmission fluid. Add or replace fluid as necessary.
2. Scan the vehicle for transmission-related trouble codes.
3. Check speed sensors. Faulty sensors can result in incorrect gear shifting.
4. Check for transmission fluid leaks. Leaks can reduce system pressure, causing slippage and drag during shifting.
5. Contact a qualified mechanic for further diagnosis and repair. Automatic transmission repairs can be complex and expensive, so an accurate diagnosis is important.