Periodic Drops on VPN Connection

I would like to know if anyone has been able to get a site to site, ipsec based, VPN working without random periodic drops using AT&T/Bellsouth with a single static or dynamic WAN addresses?  I can get a VPN connection to work, but after period of time (15-60 minutes) the VPN connection drops and even though the VPN light on the router remains lit, the VPN connection is dead.  Power cycling the 851 will bring the VPN back to life….for another 15-60 minutes.

I run a similar router config file for Charter cable and Windstream DSL dynamic addressing and they work fine.  I have tried this setup in multiple locations to eliminate the possibility of line issues…without success.  I can’t figure out what is different about AT&T and their techs have no clue.  It’s almost as if AT&T is doing something to reset the connection every “x” number of minutes.  VPN connections are perfect on AT&T if I use a “block” of static IP’s.  No drops or resets.  Naturally, that requires using their more expensive business service…  :-(

Here is the basic config…
Westell or Nettopia DSL modem set to bridge mode.  The DSL Modem handles the PPPoe connection and authentication.
Cisco 851 Router handles everything else…internal 192.168.x.x network, ipsec  site to site VPN connection to Cisco ASA 5510.

If anyone has a similar config working on AT&T/Bellsouth, I love to hear about it.  Any thoughts or suggestions appreciated…