Thank you for the replies and info. I too suspected the cable box initially, and did exchange it for a 'new' one at the Charter offices. None of them are really new anymore, they just refurb/reissue them out. At any rate, the box was exchanged after the first event. I don't doubt that the cable boxes may be at fault, but at this point i've had two separate boxes produce the same result on this receiver. To me it seems like a problem with the Denon. I don't understand why a HDMI signal would damage a receiver, but it's happening.
It seems to me that sending it back into repair is a wasted effort. It's only been a month or so since it's been back from the last repair. Yet if it's a problem with the receivers, I would think there would be MANY more cases of this happening. There's a lot of Charter customers out there.
I mentioned storms, as its been asked of this before. Don't recall much of any lightning happening though, just rain and thunder. But I agree it seems far fetched that a power surge could cause this, unless the cable box spikes and somehow sends higher than normal voltages through the HDMI port. But at this point, there really doesn't seem like a good explanation, other than the receiver is faulty.
The only thing I can think of doing is run the cable directly into the TV, which is definitely not desirable and not really acceptable to me. Or I could access cable content through a Roku, eliminating the box. Still not desirable, but an option I suppose. I think I'd rather try getting a new receiver first.