They suggested the latest firmware and got the same results. I've rolled now the latest firmware is too unstable.

Draytek asked what steps I did to reproduce the issue. So I did the Janet and John basic steps so they knew I hadn't missed anything.

They come back and said the 3910 at present doesn't support jumbo frames which is surprising considering it's clearly expected to work in a hide speed enviroment where such usage is very possible.