This is probably the most likely problem. Without a Bode analyser, finding out the stability of the control loop is difficult. Check the drive to the FET. If you can see different length pulses, but with a constant load, this loop instability and the compensation components will alter this. Board layout also needs to be impecible. You cannot build these on strip board. It needs at least a 2 layer (ideally 4 layer) PCBFluctuation in the output voltage is probably due to improper compensation.
It needs at least a 2 layer (ideally 4 layer) PCB
I don't see why?, it's a simple circuit, so easy to design as single sided - although as any PCB manufacturers (such as JLCPCB etc.) do double-sided as a minimum, you may as well go double-sided.You can, but your life will be made easier with a 2 layer PCB