Audio and video out of synch

I've had my TV set up with a home theater system for over two years.  During that time, I had only a handful of occurrences where the audio and video were not in synch.  But then starting in about early February 2012, there was a huge jump in the number of networks on which this audio/video out of synch problem showed up.  Now it happens with about half the networks I view on a regular basis.  Compounding the problem is the fact that an adjustment I make to the audio speed (I may not be using the correct terminology) for one network doesn't properly correct the problem on another network.  In essence, I have to adjust the audio speed separately for many of the networks.

Is the cause of this change something that Cox started doing in early February?  Is it a potential problem with my home theater system?  Any advice is much appreciated.