Its a signal issue.
I get the same thing on one tv.
With the new digital signal you cant tune it like the old analog system where you can have a stable less than perfect picture.
Id say at a guess something has changed between you and the transmitter or on your arial itself.
Easy fix with a mast head booster.
http://www.jaycar.com.au/Sight-%26-Soun ... r/p/LT3275Something like this.
Just make sure its digital ready