Yes I do think the DoS setting will help. If you read the posts in the link I used, there is a link back to a Draytek page which suggests this setting needs to be changed depending on the speed of your connection. This certainly seems appropriate for me as the problem started when I upgraded to VDSL. Also, it seemed to happen more when more devices were in use.

I'd like to hear a better explanation as to why as my networking skills don't go into this much detail!