Hi, I have the same problem and its frustrating as I take a lot of family videos with my N9.

One solution is prior to recording videos, I would do a reboot of my N9, then it would work fine for a while before it starts lagging again (usually after prolong use).

It's unpredictable and inconsistent lagging, sometimes within 10 secs of recording, sometimes more. Sound will continue to be recorded though with no lagging.

I really hope there is a fix for this.