I have a 3A fan hooked up to an ATX power supply in parallel with a transformer which powers my board through VIN/GND.
The ATX is gated by a relay on its DC side and seems to cause enough of a current drain that it causes the Arduino to reset.
I have place a 470μF capacitor accross the Arduinos VIN and GND pins and it seems to have stopped the problem, but I'm not too experienced using capacitors. Is this an apropriate value? I'm not sure what difference it would make. And is this a wise move as I so think?
EDIT: Here's a very basic sketch..
12V Transformer powers arduino on VIN/GND pins, other pins are 5V, GND and control for relay