It would be useful if you could determine whether it was the record or playback function which was at fault by playing the disk on another machine.

Are you making the MD recordings from the keyboard and then playing them back against it ? - if so a temporary work-around would be to deliberatey set the pitch higher on the keyboard when you record.

According to to this review of the XP50, it has a multi-function pitch and mod lever.

It's a pity that the XP50 doesn't have an optical digital-out socket like some of the latest keyboards, you'd then be able to make a recording directly via the digital-in socket on the recorder.