Certainly, causing any hardawre to heat up like puts stress on it, and lag could potentially be one result. Though I would check out exactly what your video card is doing among the scheme of things. If you're using a stock cooler, a nice after market may help out.
well if it was running fine before than i suppose that must be it
That said your gpu is below required system specs so that def isnt helping. http://left4dead411.com/news/2008/09/l4d-system-requirements/
actually my pc meets the specs just not the video video card but i play on low settings, 800*600 and get around 40 fps, lags and chokes and ping jumps, so im pretty sure its this new modem, also ive tired to play cz on a diff computer that i hooked into that same modem and it got choke and lag , ping would jump from like 20-70s lol f***ing comcast. With old rca modem, i never had this problem.