Cisco WAN :: 877 - Virtual Interface Goes Down But Not Physical Interface

I have five 877 routers connected to ADSL circuits provided by Vodafone. Each has a VPN tunnel back to a PIX.
 
Occasionally one of the sites will lose it's connection to the PIX.
 
When we check the log, we find entries like these:-

Apr  5 01:31:54.085 UTC: %LINEPROTO-5-UPDOWN: Line protocol on Interface Virtual-Access2, changed state to downApr  5 01:33:19.344 UTC: %CRYPTO-

[Code].....
 
As you can see, the physical interface (ATM0) is not being reported as changing state to down, neither is the Dialer interface.
 
When the router is in this state we have to SSL to the public IP address of it and manually restart the ISAKMP SA.
 
When the router sees the ATM interface go down and subsequently come back up, the VPN connection to the PIX also recovers.
 
So - in a long winded way I think I'm asking....why does the Virtual interface go down and is there anything I can do to stop it happening?