LG 42LB5DF turns on by itself and the channels...

Question

I bought this tv second hand and all has been well but I got home the other day and the tv was on. I started watching it and now the channels keep going up in numbers. Its not the remote or the buttons on the side of the tv. All presses in and out with no problems. Can you plase let me know what to do and what part I might need to order to repair it. Thanks! If I unplug it for a little while it will work with no problems for a little bit. Not sure what the cost would be to repair it.