CDJ 2000nxs lags when using any buttons around screen, sometimes buttons don't work at all

While using the units the other day, I noticed that one of them was taking a few seconds to register my commands, it never dont this before... I used the selection wheel to choose a track and it took 1/3 seconds to actually perform the action, this was via LINK however i could have used it on DISC and USB it was still the same thing... selecting a song is impossible some times, but other times it will select after a long delay ofcourse ... updated firmware to 1.22 still having the same problem. All the buttons around the screen perform in a similar way. I don't suppose anyone had any issues like this before? very strange how it just suddenly happened... 

any information regarding this would be very much appreciated

thanks!