My ventilation system is stuck on boost mode. How to fix it?

I have a heat recovery ventilation system from baxi instaled at home for 5 years now. It opperates on normal speed in normal conditions and should only turn on boost mode if we manually operate the switch or when the level of humidity within the house gets to high levels, as it also has a humidity sensitive device instaled. For the last few months it has been opperating normaly during the day, but as the night falls it starts to switch to boost mode constantly and off for few seconds and on boost again for a minute or so. It is really disturbing as the system is quite noisy and we can feel the draft of air in the house. We clean the filters every 2 months and had just done now, but with no improovement has shown. We try to keep windows open and observe, but at one point it goes on boost till next morning. What can be causing that?