If it happens on all context on this channel then it could be a bad signal of that channel. Try resetting the coax cable and rebooting the STB. If it happens on HBO Family on all your STB then call verizon Support. But try that coax reset first, it's a quick and easy fix.
Disconnect and reconnect the coax.
Is it all boxes or just one?
If all, start at ONT and work up to the first splitter.
If it is just one box, start at that box and work back to the first splitter.
I've done that already. And I had this problem back when I first got FiOS back in the fall, just with a different station. Now there's another station it's happening with. I think I've learned that intermittent pixelation really can't be fixed. The techs don't even like working on it, mainly because it's kind of hard to track down if it's not happening all the time. And even if it does happen when they see it it's too complicated to go through everything. I don't think FiOS works like regular cable, but I'm not sure. But of course cable had these problems too. Is ANY system perfect?
Do you have more than 1 TV? If does it happen on all of them?
Both HD and SD?