strange thing happens every night

this strange thing happens every night on bbc1 itv1,2,3,4 bbc3 bbc4 watch and a couple of others not the plus 1 channels.
every night about 9:30 to 12 some times longer the picture breaks up and lose signal any one else know the reason please.

500s commando image,multi sat config fixed to a motorized 1.2 channel master. i have 3 boxes fixed to this dish and it happens to this box only.