This problem is caused because the guitar in question is not outputting the correct pitch. This typically occurs because of one of two reasons. If this is occurring on a particular fret and note then chances are that you have a 'sweet spot' on your guitar that is outputting a tone that the GK-2A pickup is interpreting as an octave above the intended pitch. If this is occurring on a particular string or range of frets, the chances are that a harmonic of a note is being accidentally triggered for that string or group frets. This effect can also occur with open strings. When the GK-2 or GK-2A pickup sees this new pitch, it assumes you meant to play a harmonic on the guitar and accordingly matches it with the appropriate note one octave up. This is primarily caused when a string vibrates and hits a loose or high fret and in turn creates a harmonic. Whichever problem you are having, typically the solution is a 'fret file'. A 'fret file' is an operation where a technician will file down the frets of your guitar so they have more consistent height. This operation should be performed by a qualified guitar technician. Raising the action of the strings can also yield results.