Low water pressure and temperature in shower every fall/winter: how to fix it?

I posted this last week in .plumbing, but didn't get a reply. Maybe i'll have more luck here.

I live in an apartment in Spain. We have a gas water heater, cranked up to its maximum temperature for hot water (60C / 140F) and 75C / 167F for the home heating (radiators).

The shower is the water outlet furthest away from the hot water tank.

I'm not sure if one's causing the other or not, but every year, around November, when it gets a bit colder (and I really mean a bit, it's 12C/54F at night and 20C / 68F during the day), the water pressure in the shower drops quite significantly, and with it how hot the water gets. In the spring/summer, I put the temperature control right in the middle, which is plenty hot for me. Now, I have to put it at max hot, and it's barely warm.

Any clues how to troubleshoot this?

Thanks