Not a "diagnosis" but.....
by Samsung_HD_Tech - 1/13/09 5:13 PM
In Reply to: Samsung LE40M86 LCD TV changing channels by itself! by sskssk73
What I might suggest in terms of diagnosing this issue is that the symptoms suggest that the physical panel buttons on the television set are connecting even when they're not being pushed.
Both the channel changing and the power button turning the unit on, to me, says that there may be something with the keypad, rather than the motherboard. If this starts happening again, try to interrupt it by pushing the channel in the opposite direction or tapping the channel up/down button with your finger.
That repair, even out of warranty, should be considerably less expensive than replacing a PCB in the television. You might see if maybe the service center would diagnose it for free and then at least weigh your options if that's the cause.
Keep me posted.