M50 88 keys not working

[nico88]

My M50 is doing some strange things. A few days ago some of the keys seemed to stop working, then would work albeit slowly. Unplugged everything then left it overnight, next day everything was fine. Played it most days including this afternoon, and I edited some sounds and created combis etc.
However, tonight I have gone back to the keyboard to find every C, E and Ab in each octave are not responding. Any thoughts?