Yes, I understand all that. Basically you need to isolate whether it is the cable box or the display. If it is the display you should try to isolate whether it happens only for external input or whether it happens on internal tuner also.
Even if you have Comcast digital, your cable from the wall still carries analog cable (unless you are in a couple of test areas around the country which got rid of analog completely) If you plug in the coax from the wal to the antenna input you should be able to setup analog cable and test whether this happens on the internal tuner.
The reason I'm suggesting you go through all this trouble is because as far as I know, nobody has ever reported anything like you describe. It is just not a common type of failure. It is very strange it would only happen in zoom mode and not the others. I don't even know if it is the TV or something else because there is no past history to go on. Therefore I think it justifies the extra investigation to narrow down the problem.