I have an issue on my 2850Vn where the VPN tunnel into my employers system connects with no problems at all. However what I am seeing with the data, particularly inbound to me data, is that I get traffic for 10-20 seconds and then it just stops. After a pause of maybe a minute or so I might get another burst but I might not. The tunnels never drop abd it seems particularly prevalent when connected to SharePoint 2010 systems...though why that makes a differebce I don't know. I cannot pull files of more than a few hundred Kb via the VPN without an issue.

I have also noticed that when running a speed test, the inbound rate is stable, but the outbound data stream runs for a few seconds, pauses, suddenly leaps to an improbably high rate and carries on.

I've reverted us back to a 2820Vn and the problem has disappeared. Unfortunately my outbound data rate has dropped 30% with the change....the 2820Vn has always done this and Draytek looked at it under warranty and said it was fine! Both an ancient 2600 and the 2850Vn have the same in/out rates as each other, but not the 2820Vn. No idea why but.....must be a 'feature'.

I have updated the firmware, but no difference. I'm on ADSL (no fibre here!) with a 5.5Mbps inbound and 400kbps (2850) outbound rate. Line att is around 34dB, SNR is 8-10dB. The 2820Vn shown no outbound data rate instability compared to the 2850Vn. It's a nice steady constant rate.

Has anyone else come across the VPN traffic issue? If so what was the fix? I suspect the 2850Vn itself may have an issue and as it's under warranty I hope Draytek will have an answer (or replacement) but I thought I'd check with the knowledgeable group here

Turn off DoS defense on the router if you've got that enabled - your description suggests the UDP flood defense is being triggered and coming back each time. Syslog output from the router would confirm it.

Thanks, I'll give that a try. Draytek are themselves going to have a look at the system remotely and see what they can find too.