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.

I wish I had seen this the thread before.
Uninstalling plex media server seems to have fixed my router crashing. Its only taken me two evenings, binding ips, changing dns, doing away with the wire and trying WiFi, updating drivers etc. After all that it looks like its caused by software.. Hope my IP profile doesn't take too long to recover.
2860n 3.7.3

I am having the exact same issue, 2860N rebooting every few minutes. I downgraded to the 3.7.3.3 firmware from the latest beta and hard reset the router in the hope this would solve the problem.
I had installed plex server a few days ago and this ties in perfectly with the time the Draytek started acting up, I wish I had seen this port earlier too because my sync speed has dropped to 47Mb/17Mb from 70Mb/20Mb because of all the reboots. I hope Draytek release a fix for this soon.

UPDATE:- uninstalling Plex Version 0.9.9.7 seems to have stopped the router rebooting.

I managed to do a little more testing since the upgrade to Synology DSM 5 requires a version of Plex which crashes the Drayek.

Current state:
Draytek 2860N+ running 3.7.4.1_RC4 (for a different problem)
Synology 1813+ running DSM 5.0-4458 (not update 1 yet)
Plex 0.9.9.7.429-f80a8d6

---->Crashes the Draytek routinely.

Actions taken:
Disable UPNP on Draytek
Disable DLNA on Plex

---->Draytek stays up

Reported this to Draytek support who confirm they know about the problem and working on a fix:

This is an issue that has recently been identified and reported to the
firmware team so I'll send you an update once we have more information
on a fix for it.

Progress and maybe an end in sight!

There is Official firmware download of the 3.7.4.1 here
ftp://ftp.draytek.com/Vigor2860/v3.7.4.1/
& at main web of Taiwan

Im also having this same issue.

Does this firmware upgrade resolve the problem?

I sent a support request into Draytek Uk over a week ago but still havent had a reply!

Shocking service!!!