Rick,
I have the same issue with my HD signal through cable. Being in Canada I of course do not have the same provider as you (I have Rogers) but this is a sporadic symptom I have become accustomed to. It is very brief and doesn't bother me for casual viewing. My in-laws have the same provider and the same issue as well with none of the same equipment as me, so I have just made the assumption it is signal breakdown from the provider.

Dan
On-Wall M5HP LCR, QS8 & EP500 in 7.1