You could check the signal strength on your end by entering the the "Services" menu. Both signal strength an quality bars should be at least 75% to avoid problems.
If this is o.k as it is a Grundig it may be the power supply unit requiring a reliability kit of components (some companies were supplying the complete unit replacement).
Ensure connections are sound and try the box at a friends to see if causing problems there (an over night test would be required if the fault did not show right away as the box may require time to heat up before faults show up.
If the box is faulty you could have it repaired or purchase a refurbished one from this company. Incidentally I think this company do a fixed price repair regardless of the fault.
www.mces.co.uk
Alternatively contact sky and threaten cancellation and they will usually offer a deal (the deal depends on who you speak to).
sparky
December 2005