What firmware are you on?

EDIt

latest firmware has this option

If you don't have that option then you can ignore that suggestion

It will be the DoS Defense - UDP Flood defense - you can confirm it by looking at syslog output from the router when it happens

Replying in this thread as I'm seeing a behaviour which looks to be closely related to the OP's problem.

I have a Linux server sitting behind a 2830 (FW: 3.6.8.2) which has been showing DNS lookup errors in the mail logs.

While this server is set to use external (WAN) DNS for it's lookups I'm seeing rDNS responses coming from the Draytek's DNS cache instead of the external source.

When the DNS cache is empty name resolution is OK, but when an address is cached rDNS responses are corrupt with dig reporting ";; Got bad packet: extra input data"

The fix is, as suggested, to have the DNS Cache on the Draytek set to as low a value as possible, but being able to disable DNS cache would be even better and better still get a fix for what looks like a bug.

A