Thompson Sky HD boxes are known for this fault and many others due to Power Supply Unit (PSU) faults.
If you check the signal strength in service when the fault happens does one feed show ok and the other shows no signal.
I had this problem and repaired the PSU with spare parts brought as a pack from satcure.co.uk look for the kit RELKIT33j also loads of advice on this problem on avforums.com
Phil
July 2010