I've had the same problem, and I think it's "Anti-mashing." As a relative noob I think I still give more power pushing down on the pedals than pulling up, so when I sprint I add more power to the upstroke and things get unbalanced. Dropping the cadence in a sprint and focusing on forward/backward pushes instead of up/down pushes/pulls has helped a lot.

Does that make any sense? I hope so.