Switching Mode Power Supply Fluctuating Output
The regulation of that configuration can be poor, as it depends on a tight coupling between N2 and N3 and how closely does the voltage on C3 follows the voltage of the changing load. Given the 36k &amp; 1uF load on that feedback circuit it is likely to be too slow to respond to load changes, and too sensitive to initial transients on the transformer coils. 

You can try by reducing the 33k resistor and/or 1uF capacitor. And/or adding a load directly to N3 to reduce the influence of the switching transients.