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.
I purchased a Samsung LE40M88 LCD Tv over a year ago, after three months of having it, I got a problem. When i would switch it on it would flick through all the digital tv channels for about an hour and then settle on a channel, and I could then operate it as normal. There was nothing i could do to stop it. As it was under warranty I had an engineer come out and fix it, apparently he replaced the motherboard! It worked again ok for about 10 months and now recently it has started doing it again. Also, if we leave the tv on standby it sometimes switches itself on and then continues to flick through all the channels so we have to make sure that we turn it off by the plug. It is now out of warranty, I know that they replaced the motherboard before - do you think i could buy one and do it myself? Pleasssssssse does anyone have any suggestions on how to fix this problem? Thank you