Freeview box ?

Sony freeview box TV an elderly Kenwood flat screen bit like its owner, old I mean not flat.
Why am I losing reception/signal every so often on channels 11 & 12 when all other services are fine its only these two that do it?. You lose the picture to pixilation then no signal. Waiting about 40mins pictures came back and within 5 mins of a perfectly good picture it happens all over again.
Its not the aerial that is an expensive digital aerial and the fact I can get all other freeview stations with no problem. Why only these two channels? this occurs regular when questioned they say you pay for receiving, so will now keep a log for the lost time to get some money knocked off my license fee for NOT receiving these two channels for x amount of time. Watch your daily for the case to come up where I’m taken to court by the Licensing authority for BBC and watch their faces on the TV news ( not freeview you’ll miss it) when I win my case and they have to refund some of my license fee and pay court costs plus trauma compensation.................I wish!!!
Many thanks for reading and taking time to reply.