MIDI notes changing pitch in there own for no apparent reason

When I’m playing midi notes in it keeps changing pitch on its own with no apparent reason. It still shows on the piano roll on logic as being the exact same note but plays completely different notes. 

So for example i pay an F note on the keyboard and it shows on the logic piano roll as being an F note. I keep playing it over and over again and after a few seconds it suddenly changes to become a C note (8 Semi tones above) but still shoes on the piano roll as being an F and then after another minute or so it changes and becomes an E note below it etc etc. All the notes around it change as well. 

This is driving absolutely f****** mental. 

Any help much appreciated.

Cheers