Probably interference. Where I live on the coast we have regular visits by oil and gas tankers. Their sides give a good reflection for the TV signals which interfere with the direct signal from the transmitter. I think different stations are affected by different amounts because not all channels are broadcast at the same power from any given transmitter. An indoor aerial should help as you can reposition it temporarily when the breakup occurs. Being only half a mile from the transmitter should allow you to use a set top aerial.
I know when a two stroke scooter is about to go down my road before I even hear it, my picture completely breaks up for about 30 secs, and always at an important moment.
And, damn, if there ain't 100's of the little ******* !