I upgraded from the Science Museum Powerball to the NSDBall model with flashing lights, bells and whistles. The new more expensive one seems to run smoother, but not as fast. Peculiar problem: I can accelerate it with my left = weaker hand to possibly a higher speed than with my right, BUT as soon as I reach the "limit"(which I doubt is very high) I find it difficult to keep the thing "in sync" and it starts to clunk and lose speed. Left hand only so it must be something to do with my technique. Any ideas for a solution other than endless practice?