Start with the simple things. The thing to do is narrow it down.
-Check the cable and coax plug that goes into the TV. If it passes thru any other devices, try plugging it directly to the TV to see what happens.
-Try also another set on the same aerial feed. Sometimes the Tv's tuner can be to blame for these problems.
-Do you have any other sets around the house on the aerial? Are they also afected? Mast head amp?
If all else fails, you'll have to get an aerial rigger who will check the signal with a field strength meter and hopefully check to see that aerial is intact, the downlead has not lost its shielding or got water in it, etc. I'd ask here for recommendations.