We have seen several clients recently complain of their internet connections dropping at irregular intervals.
It looks like these are all on High Speed ADSL connections. So perhaps it could be down to Line Flap or what ever else BT are going to call this condition.
However to improve the stability of the Voyager routers upgrade to the latest Firmware which is something like 3.01g and if you have a 220V router, and are experience issues with VPN, then upgrade in any case. (Haven't tested the last bit, but the website says it should fix it.)