xxmamakinxx:
MarkT - Are you suggesting that the MOSFETs I'm using could potentially be generating a magnetic field and interfering with the Arduino because they're too close?
All high-current circuitry generates magnetic fields, but its the rate of change
of magnetic field that induces interference, note - so frequency and magnitude
of the current and the area of any loop in the circuit all contribute. This is the
impact of Maxwell's equations.
The MOSFETs switch the current fast, so the rate of change of current is due
to them.