The B and G strings of my Gibson Les Paul regularly go out of tune up(!!) a semitone when playing and sometimes a whole tone if left over night.

Can anyone explain the physics of how a guitar can tune itself up? - please do not try the "Little pixies run into your practice room when you are not looking to turn the machine-heads just to piss you off" explanation as I have stayed overnight and watched it; and it isn't true!

Better still does anyone have a remedy?