Yamaha Motifxs6 digital keyboard doing weird things.

Hi everyone,
Recently I got a Motif xs6 in for repair. The customer complained about some of the notes sustaining even after the key has been released. When it starts to happen it effects all the keys in all the octaves. For eg: All the C’s and F#'s all octaves. It needed some other work done so I completed all of that and then powered it on to check it out.
The first night I tired it was working perfectly fine. Then I went into service the keybed as the whole thing was in pretty bad condition. The keybed was exceptionally bad the solder joints were crummy and black in colour. I cleaned the whole board with Isopropyl alcohol. Put everything back together and powered it on. The problem started. Keys sustaining and the effected keys sound very loud. I took some voltage measurements and it clearly showed that the keyboard “thinks” that notes are pressed even after the key was released. I measured at the diodes and it normally should show 5V not pressed and something below that when it is pressed.
Any way my thinking was ‘bad connections!’. So I RE-SOLDERED all the connections (more than 200!). It almost looks new now.
Again I put everything back together. Now the voltage measurements looks fine, but now what has started to happen after sometime only one key starts to go mad. There is a delay in registering the key press sound comes way too late when the key is pressed. It might get loud all of a sudden.
I was hoping that some one could help me figure this out. I have downloaded the service manual but it only shows how disassemble the unit and names of the IC used etc. It doesn’t show any test point measurement details or anything like that. There are two VR on the keybed circuit board named ‘Gain’ and ‘Offset’ the manual doesn’t specify what it is and how it should be set.
Here is the link for the service manual.
This is quite an important project for me. Hope you guys could help me out.
Thanks in advance.