I'm on my third box now due to a repeated issue. I just got a new box and already the issue has returned. I have periods where every 90-120 sec the sound mutes, sometimes the video freezes as well. I went through this about 18 months ago and got a new box. It took awhile to start doing it again but it did return and I put up with it for a few months before getting a new box. Now, I've had the new box 1 week and it's starting again already.
I'd check all your connections to the TV. Do you use component cables or HDMI or other means? You would be suprised how many people forget about a key component of a good TV signal out of the STB.
Intermitent issues could result to poor cable(s), and cable connections. I'd first try a complete swap-out of your TV cables since its obviously not a STB issue.