Hello, I'm a new member specifically to ask this question

I have a DrayTek dual wan router (Vigor 2930) connected to Virgin cable and BT adsl; I bought this to eleviate the problem of constantly getting errors on DNS that required me to re-boot the router. It switches to BT (slow) if the cable (fast) goes down - it uses ARP testing to "see" the failure.
Trouble is some days (not every day) it keeps going down all day long every few minutes for a few minutes; this causes problems switching isp in the middle of a remote session or a download.
Is this a know problem and any has anyone got any suggestions? Thanks!!