I would have thought it's more likely to be an aerial problem than a box problem.
Have you got another aerial socket anywhere in the house? If so, you could try connecting your STB and TV to this. It is possible that there is a bad connection somewhere on your aerial lead. I had just the problem you describe and found that the core wire of the aerial had broken just where it was connected to the wall socket. Easily cured by shortening the co-ax by about an inch and connecting up again.