I am beginning to suspect the STUN server itself is failing. Changing the time from 28 to 20 to 15 does seem to have noticeable effect on decreasing the length of the periods of audio-loss but doesn't seem to eliminate it.
I am changing my STUN server to a different one than stun.xten.com (find others public STUN servers listed here:
STUN - voip-info.org ) to see if it is the source of my problems.