Internal clock drift leads to ADS-B breakup

I use a Holybro Durandal with AP 4.3.3 installed. For security reasons I have installed a FLARM Atom transponder which is shown as a ADS-B receiver so I can get information about glider traffic in my airspace. I works really great as long as the internal time difference between the FC and the FLARM device isn’t too big. When the offset is too big, FLARM stops working. For that reason, I am in contact to the FLARM developers and they told me that the FLARM devices rely on very precise clock sync and that ArduPilot seems to synchronise the internal clock only once during startup and then uses the internal clock.
My Plane flies over 3 hours and after about 60-80 minutes the time offset seems to be so big, that the FLARM stops working.
Is there a possibility to sync the GNSS time more often to the FC? Maybe @tridge has an idea?