Disklavier

Hi All,

I own a Disklavier MX-100A. Bought it end 1989, so it is almost an Old 
Timer.
It has worked fine, until recently. I've a problem, only when recording. 
It seems that for one special key (middle A-flat) the pressing is 
recorded, but NOT THE RELEASE OF THE KEY. As a result, when playing that 
recording, this particular key goes down when first used in that 
recording (that's OK of course), but never goes up until the recording 
ends. So if this note is used in the composition again, it doesn't 
sound, because the key is still down.
This does not happen when playing older recordings, or recordings from 
other sources. And I checked all keys, but only one shows this behavior.
Any thoughts about cause, and possible solution?
Greetings, Leon (Netherlands)