Diagnosting a 24v controller (fag speed) with hall sensor hub motor

Dear experts

I am servicing an older e-bike that has the following problem.

When powering on the system everything works normally. Then after few minutes of pedaling something happens and the motor quickly looses all power.

Notes:

-The same behaviour happens everytime

-The more time the system cools down the more time the assistance works before the power fade-out.

-The bike has two batteries and fade-out happens with both of them

I am thinking that the problem is either on mosfets or in hall sensor.

The controller uses FAG speed sensor. Could the sensor have a fault that would cause this kind of behaviour in the system? Personally I would not bet on this

As these 24v systems are rare I do not have good collection of parts for cross checking.

Would somebody be able to have a strong guess about the source of the problem based on my description?

I would really like to be able to help out the older guy who owns this bike

Warm regards,
Kristian from Helsinki, Finland