I recommend that you first record a "smv" movie with SNES9X first, before you pick the AVI recording.
It's way more reliable that way and the movie will stay fresh and 60fps-ish.
When you finished recording the smv, then you can play it (turn on Read Only to be safe), and then quickly pick AVI Recording. The emulator will do the work for you, while you look at your beautiful playthrough. Stop the AVI whenever you want to stop it.