That is normally a symptom (no list of manufactures) of the box beginning to fail. I suspect you may soon get a message along the lines of "this box was not designed for use in the USA".
Obviously worth trying a factory reset first, but I think you will find it is on the way out. This is actually a known design fault, so you may be able to get a new box even if it is not under warranty. But it will cost $30 to find out.