How can I diagnose an intermittent router failure in a remote office?

It's a Draytek Vigor 2820, the only way I can tell it's stopped working is when they ring to tell me they can't dial in. On site diagnostics (asking users what lights are showing) show that the router has lost it's ADSL connection which means that I can't connect to it externally. Reboots do not fix the problem. As soon as they plug their spare router in it works straight away, problem is that it doesn't do VPN.

If left for a couple of days and plugged back in it works fine.

Any ideas?