Appreciate both of your help. I have checked the connection and the AVR speaker settings, and they are OK. That's why the center channel audio is fine on all streaming sources and most cable programs.
On the few where it cuts out, it is still present, but at a level you can only hear it a couple of inches from the speaker, and the center channel - often dialog - signal is weirdly redirected to the front L and R. If I record a program on the DVR with the issue, it remains a problem each time I play that recording. And, finally, there is no obvious pattern to where the problem will arise... e.g. I watch Dr Who on BBC America and had the problem on one episode, then not on the next.
I will shout if I find a solution.