This is definitely a STB issue. I have 2-STBs, one with a DVR, the other is the basic STB. Never have a problem with the DVR STB. I've sat and watched multiple videos, both long and short, and have never had a problem.
Now, on the other hand, the basic STB freezes every time I use it. Rebooting cures the problem for a short time, then it freezes again. Have called Verizon and gotten no solution.