I turn on the TV and seconds later it turns off on it's own.

Sounds stupid, but where is the remote control? I had a weird issue a few years ago where my TV would turn itself off constantly. Turned out the remote control 'off' button was wedged down.

Try covering the IR receiver with your hand and turning the TV on (make sure the IR window is well covered and no Infra-red could leak in!). If it is ok then something i transmitting an 'OFF' signal. If TV still turns off on its won then sounds like it could be quite ill, possibly dying.