Did you see pixellation in the previous box before it broke ?
Is it possible to try a signal booster ?
Do you have a single cable direct to the aerial or does this go through any splitters etc?
OK, we need to try to rule out as much as possible interfering with any signal.
As this is a new box and the signal is pixellating, the next option would be to have one of our engineers out to look at this, but if the fault is caused by your own aerial cabling etc you could be charged for the call out.