It looks like your mosfet is on when the MCU I/O pin is either low, floating, or pulling up weakly. Be mindful of that during MCU startup, sleep, and reset.
Slow switching might be an issue. In particular, the IRF3205 datasheet figures for switching time assume a 10V source and 4.5ohm gate resistor, for a max current of ~2.2A. Q2 is probably not giving you more than 200mA, due to the way 2n3904 gain drops sharply at higher collector currents. Ditto for Q1. There might also be additional delay in the mosfet off->on transition if Q1 enters saturation.
You probably don't have to meet EMC requirements, so upping your switching speed will only help.
What does your control scheme look like? Is it continuous conduction mode or discontinuous conduction mode?