Although rather vague of a question, it typically comes down to a poor connection. This could entail any of these things; 1.) a poor cable, 2.) a poor fitting for cable end, 3.) a cable that has exceeded its "bend radius" and has a kink or possible break 4.) a loose connection from moving equipment or 5.) a power issue with either the tv, receiver or the electrical outlet it is plugged into.
First thing I would do is check all wires and connections... check they are firmly seated in the correct ports and not old or worn out.
Second thing I would check are all the power connections...
Third, try the tv in another location if applicable to see if it is possibly local interference. If it works fine in the new location, power conditioning may be required.
If the problem still persists, the first thing i would try is swapping out tvs if brand new, and if its not a new tv... well... its time to visit your local future shop.
Cheers and good luck
