Upgraded to 3.8.4BT from 3.8.2.3BT and now getting 'regular' reboots - system stays up from 10mins to a couple of hours.
Enabled syslog to USB but can't see anything in the logs.
Also updated to 3.8.4.1BT now but same issue.
Any ideas for what to look for?

Not syslog, but there's a telnet command (which escapes me) which gives a reboot explanation code (which you can't understand but support can).

I am seeing the same problems on my 2860VN+ is there any update on this?

Contact support so that they can look at it and investigate. http://www.draytek.co.uk/support/techquery

They'd be able to run through troubleshooting with you to investigate.

Thanks, I've opened a ticket.

Are you connected via ADSL or FTTC ?

It may be worth in the WAN settings changing the DSL mode from Auto to the relevant type.

The Draytek reboots when changing the mode, so it's possible that this is being caused by something either on the line or in the 3.8.4.* firmware.