Hello Jean,
I cured a similar type of problem on one of my earlier keyboards (think it was a KN3000) by fitting a mains filter in the supply to the keyboard. You should be able to get one of these devices in Auz, from an electrical retailer. In UK, we can buy a standard mains outlet socket, already fitted with a filter and just replace the socket which is used for the keyboard.
The fact that the fault refuses to show up at your dealer's, would suggest that the problem exists at your home.

[This message has been edited by Bill Norrie (edited 05-28-2003).]