Headphone mode on X-fi causes jitters in source

Hey guys, I have an x-fi sound card. When i play games on the source engine and put the settings on headphones mode, the game start to freeze at a few seconds at random intervals which is really annoying (its like when you are lagging you freeze and start moving again). But once I shift to 5.1/6.1 mode on the settings all of these problems disappears. I have tried many other games such as bf2142, c&c3 on line and on headphones mode and they dont display the same problems. Do you think its a problem related to the source engine and the sound card drivers itself? Or should I reinstall the drivers, OS, or counter strike???