Official comment
Hey Tom,
There are a couple of possible causes:
1) Intel Turbo Boost (click here to read more on disabling it).
2) Background tasks / applications hogging system resources; check your activity monitor to see what other programs may be causing interruptions.
3) Poor motherboard design causing noise between the USB interface and other system circuits (such as the battery charging circuit). Sometimes this can be fixed with a ground-loop isolator, but not always.