Speeding up the video means dropping frames, doesn't it? With a specialized subject such as your spinning object, the missing frames might be more obvious than with a "normal" subject. Try fiddling with the "nature" sample.
Try working directly on the timeline. Put the cursor at the end of the video clip, hold down the control key while making like to pull the end of the clip, and experiment. That's faster than doing it in the Power Tools window.
Jerry Schwartz