After tv is on for a while the screen will slowly go back and forth from light to dark. Then sceen will almost look like its in thermal imaging.

The screen is not working anymore and the built in light inside of the screen is burning out and the screen needs to be replace on the The TV's screen is either Dark or blurry picture is most often caused by the result of picture setting. If the screen is still dark after turning off Energy Saving, click Back on your TV's remote to return to the Picture menu. From the Picture menu, select Picture Mode Settings. Select vivid for the highest brightness, and most vivid colors.

Back - light problem. Check and replace damaged component/s at its back-light inverter board, or replace this board as card basis. If you wish to get some details; check the site linked here. Surf the site with patience. Pull up older posts. It will be best to replace the inverter board as card basis.
http://electronicshelponline.blogspot.com/
Some model sets; especially LED back-lighted types; the back-light inverter circuit will be part of the main power supply board [SMPS]. In such cases, the power supply board might need replacement.

By go dark do you mean it turns off? If not, LED lights on the front of the laptop continue to stay on, but the screen goes dark, this could mean the backlight of the screen is going bad. It might be intermittent, but, a screen consists of two elements, the display itself, and the backlight which is what makes the image bright. If you bring it into a well lit area when it is dark and tilt the screen back and forth, you might see the faint image on the screen. Sadly, screen repairs are expensive, easily exceeding $300 most of the time. It is often a better idea to replace it rather than repair it.

you may have a back light inverter board bad.
undo the back and check the power supply for bad caps in the inverter area , or check the inverter board

When the TV is on, if you look at the screen and can see dark images, then you will need to replace both the inverters. The inverters drive/power the back light inside the LCD to show the images. You may have to try and different channels and or inputs to truly see if you can see dark images while the TV is on. If you are hearing audio from a channel or an input then you should be able to see the dark images if just the inverters are bad.
If the screen is dead black without any dark images, then typically your LCD screen/module is bad.
If the TV powers up (power supply is good), you have audio and the remote will work with the TV (the main board is good), the only parts left is the inverters or the LCD screen/module.
I can get you LG part numbers if you need them.

You may have a defective CCFL backlight, it may have the following symptom: Screen flashes on and off or displays reddish pink hue. Picture stays on in red and slowly be come normal. Picture flickering with dim display or appear black. Picture flickers on and off. All these symptoms indicate that the LCD backlight lamp (CCFL Lamp) has reached the end of the life and all you need to do is replace the CCFL lamp.
To determine if the video card or the LCD is faulty, connect an external monitor to your laptop and power the monitor up first then the laptop, if you see the normal Windows images then the video card and laptop is OK and the problem is definitely the backlight. Dim image and/or dark display on the laptop's LCD screen indicates a faulty LCD backlight. It could be a faulty inverter that supplies high voltage to the CCFL lamp or it is the CCLF lamp is nearly burnt out, usually the CCFL lamp fails before the inverter.
The inverter can be replaced easily but the CCLF lamp is more time consuming and requires soldering skills.
Check out LCD Parts for service, parts, and DIY info Use this link to their WEB site: -
www.lcdparts.net

More information is required. Your backlight is lit (screen not dark) but I need to know what you have connected to the TV (your sources) and how they are connected. Does the menu/OSD (on screen display) work?

Usually if your getting it with the MENU then your connection is weak, are you going through a VCR or some other type of device besides directly to the tv? Try hooking unit directly to the tv. Besides that, its possible you could of messed with the settings, go to the menu, go to advanced and do a reset to make sure no settings were adjusted, the disk has to be out of the tray while you do the reset.

the problem with LCD tvs are that they have back lamps that shine thru the LCD sceen. the job of the screen is to block the light of the different colors to make the picture. the problem is that the very thin screen can not block all the light. if the set has a back lamp setting turn down the back lamps and adjust the brightness control it should help a little. you my notice that the blacks and dark sceens look better when the room is fully lit and look much worse in a darken room.

Your problem most likely is in your high voltage circuit, and the 160 volt radial lead capacitor is probably gone high ESR or changed in capacitence, this usually happens during bright sceens or sceen change.