Vintage Synth Repair

Hi there! 

I have a Prodigy with a quite strange problem.

If you push a key, there is sometimes a crackling/clicking noise.

I checked the keys, all contacts seem to be clean. Also cleaned them 
again carefully. The unit was not played so often for the last 10 years.

The noise is not always there, and not only on a few keys, but comes 
irregularly. Sounds like somebody is pushing a light switch in the same 
room, and you hear this in the Monitor system...
But comes sometimes a few times in sequence, than again for a few 
strikes nothing... 

On all my other keyboards there are no problems.

Can anybody help?

Regards, Ambrosia