Mox8 out of tune

[Curtis st louis]

I got my mox8 2013 November it's 4 years old from amazon ,I love the keyboard working well till about two mouths ago start to give this problem ,u be playing and when u make a chord it's will keep going on like apegioos, a specially d key and a flat keys gets out of tune ,when u are running down your scales sometime u get no sound from keys and key gets real loud ,am real frustrated about this situation any one as a solution to solve this problem.