MCU Reset with DC Motor driving (PWM)
Do you happen to have an oscilloscope that you can use to look at the power supply? If you do, look to see if you are seeing an initial dip in the power supply.  You may be having a very brief dip in the power supply due to the surge current. That dip could have a fall time of a few nanoseconds. If that is what you are running into, then a 100 uF bulk capacitor has so much lead inductance and it will act as though it is not even there. In that case, you need to add something like a 0.1 uF ceramic capacitor with very short leads instead. The bulk capacitor supplies the long duration burst of current and the small decoupling capacitor(s) take care of the spikes.