What thescruff said. If that doesn't cure it...
Has your area been totally switched over to digital yet? If not the digital signal might still be being broadcast on reduced power. That makes it more susceptible to locally generated noise.
You could also consider getting your TV aerial set up checked. Do you have a back of the set booster amplifier? That might make digital TV reception possible but the booster will also amplify any local noise. Some booster amplifiers are also badly screened and let in local noise. If the TV transmitter has been turned up to full power removing the booster amplifier may improve reliability.
Best practice is allways to start with the right type and size of aerial located as high as possible. Then use a good quality down lead. If you have several TVs consider a proper distribution amplifier (not a booster amplifier) in the loft.
If the signal is very weak and you can't use a bigger aerial then a mast head amplifier can be used but it must go at the mast head not at the back of the set).
Something else to check would any aerial extension leads or sockets you have (eg between TV and DVR). A broken wire or poor contact might not be enough to loose the TV signal but might be enough to degrade it and allow noise in. Ideally you want cable that looks more like the ones at the top than the bottom...
http://www.wrightsaerials.tv/articles/images/Coax%20cable%20quality%20fig%201.jp...