Are you on ADSL Max, and do you get connect speeds lower than 3 meg?
My Essex router does much the same random line-drop thing. All previous routers there have done the same. The same routers, if I plug them in in Norfolk, are fine. I get <3 meg in Essex (an urban area), and >6 meg in Norfolk (a rural village!).
Draytek were aware of the problem, and shipped me a "low line quality firmware update" which reduced the glitches from several a day to about one every two weeks.
The problem is basically line quality and length. The BT kit in the exchange glitches because of noise on the line; the router doesn't know to restart and reconnect, so it sits there not working.
I don't know what the Draytek firmware did, but it improved things. Draytek offered me the option of sending the router back for a thorough check, but since the Netgear, Linksys, and several others did the same thing, I concluded the problem is with BT and haven't bothered.
If I'm home when it does it, I just switch off and immediately on again.
It gets to be a pain when I'm away and want to connect via the VPN. I solved that with a timeswitch that turns the router off and on again at 4am every day. If I can't connect today, I should be able to do so tomorrow.