shane, you have some options.
1) you can cover the bottom of your oven with heat retaining media ie firebricks, steel,? Once the oven and the media have reached 400 degrees, then the oven will recover/return to 400 degrees faster.
2) You could build/buy a PID to control the oven temperature
3) you could spend $50 to $$$$$ and get a new convection oven.
I bought a used Oster 2 shelf convection oven at a thrift store for $20 and can bake 16 lbs of bullets at a time in it.