VPN at night

I have a strange problem making VPN connections from home to one of my clients. I cannot successfully connect during the night! Daytime connections usually work, but night-time connections get a ?Error 800: unable to establish the VPN connection? message. It is as if the computer cannot be reached or woken up. At home I use a D-Link 624+ router. The client uses a Draytek Vigor 2600. I have another client with the same router/settings and it works fine. Firmware is up to date. The computer at the client is a fairly well specified Dell ? a Dimension 4600. I can connect the other way around (from client to my home) and I can leave that connection on overnight without problems, so it isn?t a matter of the phone lines being cut off at night. The only thing I can think of is that I need a special firmware upgrade for the Draytek 2600. The ADSL line I have is through BT Connect, and I am not using the ?modem? that they supplied (because I need a router with a 4 port switch). I notice on Draytek?s site that they have an alternative firmware for UK connections if they are through BT. Does anybody know what this is? Is it the answer to my problem?