The best way to diagnose these problems is to turn off all "effects" and extra video processing, set Vsync on, se if the problem is gone. If so, then you turn on the extras you absolutely need, until the problem comes back. Then you know the limitations of the system.
Another angle to try is reducing sound quality to 22.5
If your machine is Windows based, some options you might turn off will make things worse. In general you want DirectDraw on, and depending on your hardware Direct3D might help or it might hinder.