Possible reasons:
Problems with sensors:
*Faulty Throttle Position Sensor (TPS)
* Faulty automatic transmission input and output shaft sensor
* Faulty vehicle speed sensor
Mechanical problems with automatic transmission:
* Clogged automatic transmission oil filter
* Low oil level in automatic transmission
* Wear of clutches or steel discs
* Torque converter malfunction
* Jamming of spool valves in the valve body
Electrical problems:
* Break or short circuit in the automatic transmission wiring
* Faulty automatic transmission control unit (TCU)
Diagnosis and elimination:
1. Make sure that the automatic transmission oil level is normal and there are no leaks.
2. Check the condition of the TPS sensors, automatic transmission input and output shafts, and vehicle speed sensor. Replace faulty sensors.
3. Drain and replace the automatic transmission oil and filter.
4. Perform computer diagnostics to identify fault codes related to the automatic transmission.
5. Check for blockages or leaks in the automatic transmission hydraulic system.
6. If necessary, repair or replace faulty automatic transmission components.
7. If the problem persists after replacing the faulty components, check and replace the transmission control unit (TCU) if necessary.