I have seen on multiple Draytek models with the same / similar Broadcom
Chipsets that are running older firmware this same issue. After upgrades
to the later firmwares it seems to resolve it.
I remember back in the day when there were upgrades from the 7300 ASAM
to the 7302 ISAM; a lot of the old D-Link 602 modems that Telecom/Xtra
sent out to most of their customer base needed firmware upgrades in
order to resolve similar stability issues.