Minecart movement stutters at top speed

When travelling very fast, minecraft speed stutters, sound clicks and movement appears slowed.

Cannot get to repro in new worlds, only my own single realm world.

In a nutshell, when minecart speed tops out (either by passing 12 or more consecutive powered rails, or by speeding down a large incline (24 or more cubes of downhill progress) followed by three or more consecutive powered rails, speed stutters, and sound hiccups. Please use my realm (realm user planet_boy@hotmail.com) to repro if that helps. I can also send video shots. (In fact, watch this space for a video upload).