Switching TV boxes on my home TVs

For over a week my bedroom TV was freezing and pixelating constantly! I tried everything, tightening cables, restarting box, etc. I spoke with 3 people in the "advanced tech" dept. and got 3 different answers. I took the box to an Xfinity store and exchanged it, along with all the HDMI cables. The bedroom TV with the new box did the same thing. Yesterday, I swapped the smaller, new box in the bedroom with the large box in the living room. The bedroom TV works fine! The living room TV, with the new, smaller box, not so much. It's not freezing & pixelating as often, but it's still happening. Can I return the smaller box I just picked up 3 days ago and get another larger box for the living room? The problem is obviously the new, small box, since the larger one works on both TVs. Any suggestions would be appreciated!

Thank You