I experienced the same problem spodadically on certain channels. In other words, the channel I'm watching is all jittery, other channels are not.
But I found that if I hit the rewind button, go back 10 or 20 seconds, the problem goes away. I tried explaining it to my wife, and the best explanation I came up with is that since FIOS in a digital format, newly arrived data packets are not well organized whereas those from 20 seconds ago had time to organize themselves, eliminating the pauses.