how to solve "DSL internet connection when connected to LAN"?

Hi,

I am having a problem with my current setup. I have 1 Dedicated Line using a Cisco router 1841 and 1 DSL connection using normal router.

Two different ISP are configured with different gateways. When I use the first gateway using the Cisco router, my internet connection are all fine. But when I used the other gateway with the normal DSL connection it seems that my connection are dropping (intermittent connection or no internet at all). Given the users on my LAN are consist of 20 clients only.

I really needed to find what are the cause of this issue. I am thinking issue started with difference between modems. the normal DSL line is using a small normal modem hence the Dedicated line is using a bigger modem.

thanks