technics pr80

Hello,
As you can tell, or maybe not,
I love Technics Digital pianos.
I Think they are the best pianos
ever made, but that is not the
subject of this post.
I recently acquired a Technics pr80
fo FREE.
It works but has one issue.
When I play a key sometimes it
sounds properly, but if I play it again,
I get a soft chh... sound almost like a soft
cymbol sound instead of the piano sound.
Then if I play another key sometimes I get the piano
sound or sometime I get the chh... sound.
This does this on any key and on any instrument
setting and through the speakers or with head phones on.
I can't get it to play any instrument sound properly
EVERY time I play any key.
Any suggesions???

Thanks

p.s.
Is there a user's manual or a service manual available???
I searched and found a pr60 service manual, but not a pr80 users or service manual.

Thanks