I've shot many, many, much faster pans and quick action with my FX7s and never had this problem. So I'm not sure it's the camera.
Stephan's advice about encoding seems spot on. We'll know more if you can answer the questions I posed in my earlier response.
Sony HDR-FX7 capture quality problem
Re: Sony HDR-FX7 capture quality problem
Doughie - true, I confused with the Canon 5D Mark II (in its initial release firmware).