Jerry G. expuso:
If you are using the original type transistor, also check for high ESR
capacitors. You will need an ESR meter to find these, or change all
the caps to find it. The caps can also become a bit temperature
sensitive.

Thank you Jerry, that's the first thing I checked but in this set there are
no electrolytics in the supply mains side. I checked the secondary side caps
they are fine.
The BUT12AF is not the original one but is a suggested replacement. It seems
like this is the problem but the original ON4827 might be hard to find
around here...