canuck22, an extra thing you can check is to attenuate the signal before your tuner by using a 4-way splitter before your tuner. The 7777 preamp does amplify well, but sometimes it's too much if you're feeding to only 1 tuner; that tuner may be overloading. It really depends on the tuner you have, it happened to me only with 1 specific tuner, the Hauppauge 2250. The 4-way splitter trick is easy to do. Using 2 2-way splitters would do the same thing as a single 4-way for attenuation.
I tried attenuating the signal, and it seemed to make the hiccups worse on CBS. Fox seems to have smoothed out ... even when I removed the two 2-way splitters from the path. Odd. So now CBS is the only offender, even with a signal strength in the upper 80's and quality hovering around 80%.
What's strange is that the sharper the image, the more likely it is to have these hiccups. For example, a news feed that doesn't seem to be overly sharp (upconverted SD?) doesn't seem to have these hiccups, but a network show that's clearly HD does.
I think my next step is to test with a different tuner, in case it's my iMac that's having a hard time rendering. When the iMac is doing exports (re-encoding for Apple TV2), the CPU and memory are definitely getting stressed, and it *seems* to exacerbate the problem. That might be a contributing factor. The reason I'm not totally convinced that *is* the problem is that other stations that are coming in perfectly (CBC, SRC, City, PBSHD ...) do not experience this symptom. Stands to reason that if the iMac struggles with CBS, it would also struggle with these other channels. Unless there's something about the signal or frequency that makes CBS more difficult.
As I mentioned, next step is to borrow a neighbour's TV that has a built-in ATSC tuner to test, after which I might try to tilt my antenna up very slightly to see if that helps.
Any other suggestions?