There was a service bulletin for a similar problem on the Stern Site.. It involved changing a few components from memory.
Could be a "last resort" ?
I did have quick look, but only found one for mis-wired speakers....It wasn't a thorough search as yet!
I might try to get some better speakers for it (the new ones did make some difference) (with shielding). If I can find some, I might try installing shielded cabling for speakers as well....Just as a last ditch effort
