Not sure if you need further help, but- if you haven't already- then swap out the HDMI cable with a new one. These can be purchased at WalMart, Radio Shack or Best Buy for $15-$20.
Beyond that, have the cable provider swap out their box, or at the very least- try a reboot of the box by disconnecting it from power for 10 seconds and plugging it back in.
If the issue goes beyond that, unplug the TV for several hours to reset it's processor and try again.
Your last possible cause of this problem is a communication glitch on the Mainboard of the TV- which is the component that processes all of your AV signals. In this case, replacement of that board is needed, and given the age of this Vizio set- that part is no longer available, and when it is- is a $300 repair (not cost effective on an older 50" set).
If you need further assistance let me know. Please do not forget to click the 5 STAR RATING, at the top of the page for my efforts. It is my livelihood and you can follow up at any time if needed.