Saunier duvall boiler f24e - water pressure drops only when too cold?

For most of the year everything is ok with my boiler (saunier duvall thermaclassic 24e), but when its really really cold, i wake up to a boiler that lost water pressure during the night and is off by the morning, when i really need it to work. This doesn't happen when only the hot water is on or when water and heating are on and the temperature is not so cold outside. The water in my pipes never frozes either.
I can top up the pressure to 1 bar, but its very rare that i can go higher than that, the boiler doesn't accept more pressure during the winter. Only in the summer i can bring it to 1.5 but it drops to 1 when the cold starts and after to below that when colder. The light is flashing red quite often in the winter mornings. I have to open the tap every 3 to 5 days to regain some pressure and have the boiler on, sometimes 2 days in a row.
A lot of people seem to have a problem with water pressure and saunier duvall boilers. It seems to me that the manufacturer is not revealing some info. about their product and this frequent fault.
My boiler is two years old i had engineers arond three times, regarding this problem, they replaced the pressure valve and filling cap but the problem still here.
Any help would be very appreciated, thank you.