Before recording video, go to X-Term and kill some big processes:

killall gst-video-thumbnailerd
kill `ps | grep mafw-gst-renderer | grep -v grep | cat -d ' ' -f2`

It looks like you have some videos in your N900 and tracker actively works on it. That consumes memory and other resources and it may be a cause of frame drop etc.

EDIT: don't run system media player before you start recording.

PR1.2 may fix it - it has some bug fixes related with video.

EDIT2: unfortunately, it is a price of multi-tasking and universal device, it is very easy to produce a stable recording in single task environment but it is a nightmare for multitasking.