VPN connection using DSL or cable modem

Our company’s salesmen and retail stores connect to our system using a VPN connection. There are no problems unless someone is using a DSL line or cable modem to connect. In this case, they may get connected but after about 5 minutes they drop their connection. We’ve checked time-outs, connection speeds, etc. Also, if there is a router on their end we’ve made sure it has the proper ports open, etc…
Once the connection is lost, they can connect again immediately for another 5-10 min.

Does anyone know of issues involving VPN connections via DSL or cable modems?! Connections with regular voice/data modems have no problems.

Any ideas would be great. Thanks.