These are symptoms of a satellite dish. If im right your LNB has drifting local oscillator and needs replacing.
He Rgibb, sorry for the late reply.
That's an interesting one, ha ha. Usually rain makes the aerial perform worse at the characteristics like impedance and directional characteristics change.
I'd say it is either corrosion in the connection cable-aerial (impedance) or the aerial is pointed in the wrong direction (directional characteristics).
How did you come testing that out?! Ha ha.