CDP S160 a few keys mysteriously triggering or stopping other keys

This repair is for a poor customer who's 15 year old lad is so desperate to get it fixed! Yes he confessed to spilling some gooey drink over it! - cleaned carefully first four octaves contacts and PCB board. re graphited the board in places. Cured a dozen keys, BUT C1 + F1# + E2 + C3 + G3# giving very mysterious triggers! ( and these notes well away from where the spillage occured). These five sometimes no sound, then sometimes they all work, BUT when they are all working , holding down C1 + F1# + E2 together - The C1 delays - then double triggered several times sometimes ( whilst all three are continuously held down!) When these 5 x keys decide to work, they all work fine when pressed individually !! How mad is that ??

I know rubber contacts must be okay, swapped them to working parts of keyboard - and the working parts still no problem. Examined all connectors - very few anyway,

motherboard no visual signs of damage or corrosion etc.

Can only think the channels from some keys are interfering with others (short circuit?).

Not knowing how the send signals from each note are clustered together - so hard to prove any short or crossover going on.

.  I see some questions here others have had similar problems, - and as yet unresolved?