Can the motor supply drive the motor at full speed (ie directly connected, no transistor) without the
voltage falling below the nominal value? (ie is the power source up to the job?)
Have you ensured _both_ power and ground wires to the motor circuit branch off before the 7805, that the 7805
has suitable decoupling right next to it?
I ended up swapping over to a different brand of motor and I don't seem to have this issue now.
But yes, the motor positive and negative are connected to the power supply before the 7805, and at the 7805 I have a 10uf cap before and after it as per the instructions found here: http://arduino.cc/en/Main/Standalone
maybe the caps need to be larger?
I read the motor running at full speed and it was drawing around ~400MA, both the power supplies I tested could support 900MA and 1000MA