First of all:
If still under warranty or if you have no electronic repair experience you should take the set back to the shop or to a repair person. Some of the solutions described below can be dangerous, TV's are very hazardous to service. Even when plugged out, TV's can store very high voltages.
It can be much more tricky then working on an amp, and dangerous, be warned...
If you don't want to spend too much money, and since you're describing intermittent failures, I would suggest that you open it up and clean the hell out of it, then resolder the connections to all hot parts, plus any solder points that look cracked.
Replacing parts can start as low as 10 $, and up to hundreds, so better try resoldering first.