We had the same problem with our VDSL feed, it would continually dropout as bandwidth increased and was almost unusable.

Draytek support were very helpful and after some troubleshooting they supplied me with firmware 3.7.4_RC5.

The connection is now incredibly stable.

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.