Eddy Sea

Hi... I have a Fender Blues Deluxe Reissue amp. I bought it used. I think it’s about 7 years old but it looks brand new. Best sounding amp I’ve ever owned - I like it a lot. But when I hit a G or an Ab, it goes nuts. Starts crackling and buzzing like crazy. I’ve had it in to a certified Fender repairman. He hasn’t been able to nail down the problem. Anyone experience the same problem? Know the secret?