mshh
Full Member level 6

Why film capacitor?. In your case put a film foil cap from cathode to gnd on the boost diode
It is soldered wires on board,
So you are doing an SMPS with stripboard and wires?....not a PCB.It is soldered wires on board,
Unfortunately - this is not the cause - better layout is required ...It is soldered wires on board, I think the problem comes from mosfet parasitic capacitance.