I wrote: Possibly a capacitor across the output might help. Or even
replacing the
existing capacitor.

Oops! Do NOT put a capacitor across the output. Without surge suppresion, the
surge of that capacitor charging at turn on can damage the transistor.

DO, though, try replacing the existing filter capacitor. If that doesn't do the
trick, then the supply's design is probably deficient.

To consider other possible cures gets too involved for any brief comment in
this forum.

cc: via e-mail

Bill, K5BY