The first thing we need to do is try and figure out what is actually changing that setting for you...
Hopefully it is doing it frequently enough that we can identify where it is being triggered.
To eliminate the remote or an other IR device tripping it, turn the TV on and tape a bit of cardboard over the IR Receiver on the set. It will be a plastic opaque part of the front, putting your hand over it stops the remote from changing channels etc. Leave this on and see if the fault still occurs.
If it stops, then remove the cardboard and remove the batteries from the remote, if the fault is still stopped but starts doing it again when you power up the remote then you have a faulty remote. If the fault starts again with remote depowered then there is another IR device (another remote) that is causing the problem.
If the fault continues with the IR led covered then the fault is in the set, the first thing to check is the AV/TV switch, make sure it is sitting normally and isn't sensitive to any vibration. Push it rapidly 30 or 50 times to see if any dust can be cleared out of the switch to clear it. If fault persists remove all AV inputs and test again, if the fault persists then I am afraid the TV has an internal fault with either the switch or the IR circuitry.
You will have to take the set in for service, be sure to ask for a quote or at least give a repair ceiling cost. A service centre should charge you around $50 for an evaluation, repair cost could be as low as $50 to over $200 if the IR circuitry is failing