modem lags me out when im on xbox live...help

k so first off, if this should have gone under the networking forum, i apoligize, it could go either way. k with that out of the way, my modem (3-4 years old i might add) restarts about every 15 minutes when i play splinter cell: chaos theory on xbox live, it gets really annoying and i know it has nothing to do with my connection speed because i have a 30-40 ping. So i was just wondering if my modem isnt capable at runnig at the speeds necessary to play the game and i need to get a new one or if something was wrong with my xbox and/or my game. Any help would be appreciated, thanks guys