DSL and VPN connection stability

I have two small offices that are linked via DSL and small VPN/firewall
hardware. Both offices continue to have problems with the DSL
connection going down. As soon as the VPN devices are cycled down
and restart, the connections are restored until the next time the DSL
blinks. I have tried 3 different brands of Firewall/VPN devices but all
have the same problem. Servers at both sites run Windows Server 03.
Any suggestions as to helping the stabilization????