Hello Chupo. Perhaps with the colder weather, the faulty component is
taking longer to heat up? A suggestion - if your radio is mounted in a
wall cabinet, take it out and see if it performs better with air
circulating around and through the vent holes in the back. If that is
not the case, remove the chssis from the cabinet and see if that helps.
Another thought - is it possible that the FM stations you listen to
have lower ouput at later times of the day? Perhaps you could borrow
another FM radio to try when yours fades out.
I have heard that many people in your part of the world use tube
(valve) powered audio equipment. If your radio is of this type, a new
tube in the FM detector section could be an easy fix.

John K