Dual PC Modem Connection Issue

We have two computers - one wireless laptop (Inspiron 8600) and one wired PC connected via Linksys wireless router. We are using an ActionTec Dual PC Modem for Internet connection. A problem arose where we could not access certain sites such as eBay; at first we would get the home page and not be able to login, but then we were unable to even get the home page; Explorer would time out and give error message. When I bypass the external modem and plug the phone line directly into my internal modem connection is fine and all works well. The wireless router appears to functioning fine with laptop wirelss connectivity and computer network connectivity all performing well. Why would Internet connection work fine with a direct line in my laptop, but not work when using the external Dual PC Modem? We thought the modem might be defective and so we purchased a new unit, but the problem persists. We have really enjoyed using the ActionTec modem - we've had one for about three years now.

 

Sure would appreciate any info anyone can provide.

 

Dianne