Another idea that has occurred to me is that, when I was going into VIN on the Arduino, maybe I'm just not supplying the Arduino with enough "baseline" power to drive everything from its voltage regulators. I think that pin is supposed to accept 7v-12v and I'm simply doing badly by only supplying it with 5v. Maybe I should switch to a 7v power supply to the Arduino's VIN and see if that fixes everything.