Sluggish playback on clips with reduced audio in FCPX

During playback in the timeline, video that has no audio or has the audio level reduced will stutter. After reinstalling FCPX, the issue resolved for a day or two and then started up again. Restarting the computer sometimes solves the problem temporarily. If I connect an audio clip at full 0db volume level to the footage with no audio, playback appears fine. Footage is ProRes 422.

I have deleted preferences and removed any added plugins but the problem still remains. Any suggestions as to what I can do next would be helpful. As an added note, we have another machine with the same specs running FCPX with no stuttering playback issues so it doens't appear to be directly related to the hardware. All media is coming from the same event. My machine information is below:

Early 2009 Mac Pro - 2 x 2.26 GHz Quad-Core Intel Xeon
Memory 8 GB 1066 MHz DDR3 ECC
Graphics NVIDIA GeForce GT 120 512 MB