Latest firmware here
ftp://ftp.draytek.com/Vigor2860/v3.7.3.3/
use of *rst* file needs and back settings manually from the begin... not sure if the issue solved but if there is time can give a go

Hi all,

Draytek have emailed me firmware version 3.7.4, and it's still the same issue

Disabling UPnP will stop the undesired reboots from happening.
It's not great not having UPnP, but it's a work-around I can live with at the moment.
Hope Draytek will fix this soon.