I have opened a case with tech support and will post their findings

I found that when I enabled syslogging on my 2925 a while back, it would then cause the wifi performance to become unsable. Sure I made a post on here about it. I would have to telnet to the route and restart the wlan interface. A reboot (and the latest firmware) has seen this issue go away.

No identical, but both involve syslog

I have been conversing with Pawel @ Draytek support and have made some further observations:

1. It's not only WAN connectivity that drops, I lose LAN connectivity intermittently too
2. Syslog to USB does not cause the issue
3. Changing the syslog to point at an IP instead of a hostname also does not cause the connectivity loss..

So it seems that Syslogging to a hostname is the issue (obviously not confirmed)

Interesting comment about syslogging to IP
I run a number of Raspberry Pi devices as syslog servers on a variety of Draytek routers and have to confess I'd never considered using hostname instead of IP address for the syslog servers. Never had a problem using IP addresses.
Might be worth putting a wireshark trace on and seeing what's happening when the Draytek tries to resolve the syslog hostname ?

I was syslogging to an IP address at the far end of a site to site VPN in my configuration.

Through a VPN I hope !

In which case, ensure you tell the support people as that might be relevant.