Recently i tried a different approach to improving the chain punching speed, which you can see my first attempt in the video.
The idea was instead of trying to count the punches, to punch at full speed with as much force as possible non stop for 1 minute.
This may sound very easy, but your arms get tired very quickly, keeping them moving needs you to keep relaxed in the upper arm, and to keep the elbow down and drive the punch from the elbow.
If thats not clear, then think about this: The upper arm and the biceps are not needed in this punch they will slow you down so try to put all the energy into the elbow and drive from here, instead of the shoulder. This should give you the Tricep muscle strength instead and let you build on speed and power.
so here is the video http://www.youtube.com/watch?v=fFoufVTJebo