Random picture breakup on my 32" prive' flat screen?

the picture on my 32" flat screen prive' breaks up it starts being snowy &wavey the it goes into snowy coloured lines & then blank but the sound stays the same. my signal is satellite fed (bell expressvue) .This problem happens mainly at night very seldom during the day > It only happens on my flat screen my other 4 tTVSare fine .I have tried switching boxes and making sure all my connections are tight . I happens more often in overcast evenings and if I turn the set off for a few minutes it clears up for a short time and then happens again