I created a curve on the surface of a NURBS sphere, then attached a locator to it using Animate->Motion Paths->Attach To Motion Path. When I play the animation, it correctly follows the path from the beginning to end of the duration over 200 frames that I set, but it speeds-up and slows-down during the journey, apparently randomly. It does it on playblast and batch render too, so it's not a performance issue.

It seems to happen by different amounts each time I try a new curve.
Is there anything that could cause this to happen, and is there a workaround?

Thanks