your dvi lead has nothing to do with the overall system performance. if is just a cable and has no impact at all on performance
use fraps and see if your getting big fps drops, if so turn down the settings.
if your playing on a 1080p monitor turn the screen rez down to 1600/900 or 1050
see if that stablizes your frame rate.
if not try game booster it will turn off any unneeded backround tasks.
if you have an antivirus that has a game mode use it, this will stop the av from scanning every file as its used.
make sure your net is stable. make sure your packet data is not getting lost.
all these things can cause games to lag.