Lagarith is a great codec for .avi ... makes the big ones small (file size and sometimes consequent CPU load) and looks great also.
http://lags.leetcode.net/codec.html

Try a smaller resolution also. No need to use 1920x1080 in Sonar.
Use an intermediate ( smaller ) resolution.
Also make sure the frame rate is set correctly in Sonar.

.mpg has always worked well for me in older versions of Sonar.