Welcome to Neowin

Is the modem a software modem or hardware? Random Disconnection are usually caused by having software modems. I had a U.S Robotics Hardware modem that never disconnected me, even having it on for 10 hours connected (24 Hours even). If that doesnt work I'd check the ISP you're registered to, whether people are complaining about poor service. Now as to the game, you'll need to find a nearby server close to your vicinity, at least in the same country. I got 190 Ping in CS on a local server which was really great for 56k. Other than that, there are some registry tweaks around. Maybe you could visit
http://www.tweaktown.com or
http://tweak3d.com for information on tweaking your modem and OS for optimum speed.
To make things more simple, you're better off with DSL or Cable service. Its a major boost in Ping performance. But Im assuming you cant get it or its too expensive.
Good luck!