How to read Arduino voltage
It would appear that the wrong A/D reading is due to the reduced +5V output. The A/D reference is powered by the input to the computer chip. The big question is what is causing the voltage drop. Are you powering something else
from the USB supply when the FET is turned on? Bottom line is that you cannot
expect the A/D to read the same voltage when the voltage input to the computer chip is changed.