Ours does it all the time, especially when it rains. We've had a number of aerial men in to see if by adding some kind of booster on it will help (which the most recent one has helped to be honest) but still it gets really bad in heavy rain. It's so annoying. Some times the channel goes off completely and all I get is a message saying "channel not available".
We only have BT freeview, and I'm wondering if Sky would be any better for the problem. I'd have Virgin cable if I could but their service isn't available in my area