Yet when driving a pure resistive load, the voltage looked steady, correct? Does the motor spin at a constant speed or does it act as though it's getting variable voltage?
Yes, when driving the pure resistive load, the voltage was steady. Sometimes the motor spins at a constant speed and sometimes it acts as though it's getting a variable voltage.
I'm really starting to believe it's a breadboard issue. I wired up the TIP 120, and it worked fine. I switched back to the mosfet, and it worked as advertised, and I even measured the right voltages in the right places.
A little later I will solder this circuit onto a perfboard and test it. I'll let you know what happens.
Thanks for all your help!