I have a new 3910 I configured off the LAN with a directly connected laptop. Once I connect to the LAN, after about 30 seconds at the most but usually within a second or two of connecting to the web gui interface the LAN connection drops and the router is inaccessible, even from a directly connected PC, it stops responding to ping requests too. It will eventually start responding again consistently for 45 seconds before it drops again. If I power cycle the router then all is fine again, until I reconnect the LAN port.

Any ideas what could be happening?

Cheers
Gary

I had exactly the same issue with one I had, I RMA'd it to the supplier who appear to have confirmed an issue and have yesterday sent me a new one, although I didnt request this replacement as I was wanting a refund but they never asked what I wanted to do before sending a replacement.
Ill give this replacement a go but my money is on it will have the same issue.

Ive decided not to bother trying this one, supplier has excepted the RMA again as they didnt ask me what I wanted to do.
Maybe in a few firmware updates it'll be fine and Ill investigate again.

Is this running the 3.9.1.2 firmware or a different firmware on the Vigor 3910?

If it's working when you've got a single laptop connected but stops working when you connect it up to a network, that suggests it's not handling some data it's receiving very well. You could try running it with the WAN ports disconnected so that you can isolate whether the issue is caused by WAN traffic or LAN traffic.
I think the best way to resolve this is to contact the support team and you could get "sys ver dbg" output from the router's command line interface so that they can track the problem down quickly.

You could also try the 3.9.2 beta firmware that's available on the DrayTek UK site to see if that helps.

I originally tried the 3.9.2 firmware from the UK site as it brings in the ability to change how each port behaves. I had same issue with that version and it also broke data flow monitor, couldnt tick the box to activate. It was then thinking this release was buggy I went back to official release after a full reset and again had the same problems.
I'd love to be able to debug and help further but I cant afford to have a £700 device sat doing nothing because its doesnt work.

Not sure whether this is relevant or helpful, but the only way I could get a stable connection is to use a VLAN enabled Ethernet dongle and specify VLAN 0 in the driver properties (not normally required for 0!). Obviously use one of the last 4 Eth ports to connect (P9-P12), at least initially.