2 year old LG Flat screen TV keeps freezing

First of all try to update the firmware. If its didn't help then you can go for a new mother board .
If you wish to get some details; check the site linked here. Pull up older posts. Surf the site with patience.
http://electro-medical.blogspot.com/

When the picture goes out again, dim the lights in the room. Take a bright flashlight and shine to on the screen, do you see a faint picture?

Contact LG tech support. If this has happened before, you maybe looking at an issue with the internal storage (memory) of the tv, which may require service (even though only 2 months old).

First when tapping changes the picture you have somewhere a bad connection; in electrical board or wiring.
When the screen goes blank after a while means some capacitor is bad/old.

Unfortunately this is an unrepairable problem called screen burn.

This happens commonly with old displays and can only be fixed by replacement.

More information can be found here: http://en.wikipedia.org/wiki/Screen_burn-in

You will either have a fault with the inverter board which drives the backlights or a faulty backlight in the screen. If the problem is a backlight then a replacement screen is the only cure. One or the other is breaking down after it has been opn for a certain period of time, have an engineer call out & check with a tester if it is a screen problem.

replaced main board

Your statement is confusing. Why? because you stated " This tv is hooked up to an Antenna with a Booster", then you said it's connected to Cable Tv. I will answer this both ways for you.
If your on an Outside Antenna or Rabbit Ears, even with a booster, if the signal is not good prior to getting to the antenna then a booster only amplifies what it see's and that may be as we call it, "garbage". In a HDTV the freezing can happen when that situation is at hand, however if you are on Cable and we know the signal is clean ( we hope? ) then the problem is with the software in the set and should be looked at by a service center, or if it's right out of the box then I would exchange it.
In digital technology there's no maybe, it's yes or no! If the signal is not what's required, then you get what your experiencing now, IF, there's nothing wrong with the set, and there may be. I just repaired another brand doing the same thing and it was the Main Control Module. So, I hope I helped you in isolating the problem. Please keep me posted...

HELLO to the guy saying hello ... I am on cable and have the same freezing problem ... so take the attitude and shove it!!!!!

Yes you can get this fixed and it is still worth it. It may cost $200 but it should last a long time.