I agree with willieeckslyke,that Thomson boxes suffer from failing psu's,due to the poor quality component's used(its well documented fact) and could show the symptoms you're experiencing.
However that doesn't mean it's a 100% certainty that this is your issue!!!
Dish mis-alignment,a failing/faulty LNB, faulty/damaged cabling or even RFI from Dect phones, wireless broadband routers etc,etc could also show the same symptoms.
Have we even confirmed it is a Thomson box?
So before rushing out for a PSU upgrade,firstly i'd try and beg,borrow or steal another Sky box from friends/family for a few days and see if the problem persists.
If the problem doesn't persist then you can eliminate the above,and it's likely a problem with your Sky box.
If the problem does persist, then it's likely to be another cause......