PR603M Fizzing noise

Hi there,
I have a PR603M ensemble piano which developed a fizzing noise when notes were played. I have had someone come around to repair it. He replaced the transistors on the amp board and that seemed to do the trick. A few weeks later, the fizzing noise reoccurred. He repaired again - sorted. Then it happened again! This time he used a different brand of transistors - but... it has started to do it again.
This is hugely frustrating for both him and me. Why would the repair work, but then revert back to the same fault after a few weeks? Could there be something else that has failed that is causing the issue with the transistors over time?
Any suggestions would be gratefully received before I contact this same engineer again, or possibly another one.
BTW - when using headphones, the sound is clear and as it should be.
Many thanks in anticipation.
Phil.