Linux Border Router

Hi All.. This is my first post to the forum and i am very happy to contact you.

I came here because i have problems about put a Linux Border Router to improve qos in my docsis implementation.

I Have over 500 CM's on a CMTS Cisco uBR7223 in my company connected to a dedicated point to point Internet connection (by optic fiber to a cisco router in the ISP side at 100 Mbps)... The traffic it is around the 50 Mbps DownStream and 25 Mbps UpStream in peak time. My Linux Router is based on Debian Linux and actually only make routing jobs, nothing about qos, but really do not work and i dont know why...
In Smaller conditions the Linux make a very good job, but when i put it between the CMTS and the Internet the problems begin. The speed decrease very much, many lost packets, collisions, and problems like that... The router rules in the Linux are well, as well in the CMTS...

The Server hardware is an Intel Dual Core processor with 4 GB of RAM and 2 Integrated NIC's Intel 10/100/1000... In the best conditions i have a maximum of 10 Mbps of DownStream and 5 Mbps for UpStream and the Internet goes very slow, is this problem caused by not enough hardware or incompatibility? or simply it is not so simply to get run?

This is that i am trying to do: CM's <-> CMTS <-> Linux Router <-> Internet

And this is the normal implementation: CM's <-> CMTS <-> Internet

It is possible to get work something like this? Anyone here made this before?

I know that Linux Routers can do a great job, but maybe here is a compatibility problem between Cisco and Linux. I was reading about
BGP but im not sure that this is important in this case.

I really appreciate your help.

Thanks...

Jag.