sounds like a dirty pot! You'll need to give it a clean with some switch cleaner, something like 'Servisol' or similar. You need to get the cleaner right inside the actual pot, so a little dismantling may be needed to gain access, the 'straw' that comes with the cleaner is useful for this too. A couple of squirts and then turn the control through its full range of motion a few times. Make sure the unit is disconnected when cleaning. Hopefully this will fix it.