Tv started to make intermittent screeching for some time then would stop, now screen is blank channels and volume fine.

Sounds like this:
http://www.coppelltvrepair.com/p/428/sanyo-dp42848-p42848-00-backlight-inverters-replacement-for-dark-screen-no-image-problem

Of course your TV model may be different and the inverters may be different...but the description points to the same problem.

Your Fios box seems to be faulty

make sure set not set to sap mode as this can blank audio on certain channels

What you got here is common. The issue is the controller under the volume and channel and power butoons, This is defective!! your going to need to have the tv serviced. this part cost $75-90 bucks the labor may vary in your area. Please do be considerate as you recieved this info FREE when leaving feedback and ratings today.

Btw, I’m available to help over the phone in case u need at https://www.6ya.com/expert/edwy_1a56970cf1ea4620

Tune the tv to a channel that is just blue screen. Then turn the tv off and unplug for at least 30 sec. Then plug the tv back in and turn it on. Then leave the tv on the blank screen channel go into the menu and do auto cable channel search. After the search is complete try all channels. This worked for me. My tv will not scan if the tv is on a channel that comes in. GOOD LUCK these tv's are junk .

this is the same type of problem happening with other rca models and I know it is the EPROM CHIP but why this is failing is either because of design flaw , dirty enviroment that they were made in , to much voltage or current to the chip or too much of a draw from the chip causing it to fail . Or even a random frequency from the IR sensor or its circut I'm still investigating and testing.
leave the tv unplugged for at least 3 weeks durring this time resolder any solder that don't look good , I did this and it lasted for about 6 months(always plugged in to the wall) before it started doing it again. model # 27v412t

The only way that I know of to eliminate this problem is to buy a new TV. My ILO TV has been repaired twice since May 08 and it went out again for the third time. The problem starts out as an intermittent problem.... You turn the TV on, the audio is fine but there's no picture, you will be unable to change the channel or volume whether you're using the remote control or the buttons on the TV.
This TV should have been recalled. Don't reward the manufacturer and retailer for their bad behavior, buy your next TV from a different retailer.

Is this a new problem, or has it always displayed this "X" ?

I do not see any reference on this problem and I even checked with Sony. If you are on a cable box, I suspect that is the problem.

Hello Robert, the picture tube on your TV set is exausted. Please contact your nearest Authorized Sony Service for further help/advice. After all, Sony is a good choice. The next TV set you buy could be one of Sony-s Bravia LCD. Kind regards, Arpad