I picked up a 42 inch plasma TV for free a few months ago. The owner was getting rid of it because it shuts off after a few hours. It eventually got so bad that it acted up after just one hour. So it was time to take it apart and fix it. Here it is with the covers removed. You have to remove the base to remove the back cover.
Here is the power supply board. You will need to remove it for servicing.
First check for any blown capacitors. Then turn it over and re-solder any connections that go to anything mounted on the heat sink or that looks at all worn.
If all else fails add a cooling fan. Mine has now been running over 4 hours after re-soldering the connections.