We have a Draytek Vigor 3300 using firmware version 2.6.3 and have been having an odd intermittent issue for months where port 80 will become blocked. Port 25 is also affected but some connections get through very slowly.

We have made sure nothing else could be at fault by removing the web filter and also connecting the public-facing server directly into the Vigor but this problem still happens.
We have also swapped the Vigor as we have three of them.
We have factory reset and set up the config from scratch.

Has anyone ever had this problem or have you any idea how to resolve it?

Thanks