When driving on a highway when the car reaches 120 kmph it automatically engages in overdrive, reduces speed

Welcome to 2CarPros.

First, please understand this vehicle is not offered in the US, but I will try my best to help. Now, at the speed you indicated, it is normal for the transmission to shift into overdrive. However, you shouldn't lose power. Is that what happens? And whey you say that you lose power, do you mean the engine stalls or becomes non-responsive to throttle pressure?

Also, is the check engine light staying on when the engine is running? If it is, you really should scan the computer for diagnostic trouble codes. They will point us in the right directions to help identify the problem. Here is a quick video that shows how it is done:

https://youtu.be/YV3TRZwer8k

I realize most people don't own a scanner, but often times a parts store will check for codes for you or lend/rent a scanner to you. If the light does stay on, this really should be step one.

Let me know.

Take care,
Joe