I am wondering if you have found that DSL is better for Quake III then cable modem. If so I will switch to that service which has now become available for the same price. If I were you I would take a look above first and check out the article I suggested to Brent. Normally there are bad hops that haunt your connection. Armed with this information, you can call your ISP and demand action based on an informed complaint.
Cable is also notorious for packet loss.
Cable is a huge Network. All those users are crowded onto one big network and you are sharing their bandwidth. DSL is usually better because depending on the DSL service you choose, it is a dedicated line. So you will have to call your local DSL provider to see if you qualify. The modem configuration can matter because sometimes there are modem init.