Yamaha P-45 - Muffled sound sometimes

by Dave
(Illinois)

Most of time the sound is loud and clear, but occasionally, if I slide the volume down to 0 and then back up, the sound will be muffled somewhat and sound cloudy. Tried it with headphones when this happens and the headphones also sound muddy. If I turn it off for awhile then return later, the sound will usually again be louder and clearer at all volumes and with the headphones. And it is not just my opinion. Other members of my family who play also notice this. Any ideas?