Well the MOSFET switching the resistive divider isn't going to work well, its neither a high-side
nor a low-side switch, there's no guarantee the 2N7000 is switched on properly.
Are you clearing the ADC state after power up (take a reading and discard the result)?
You are assuming the problem is hardware and isn't in the code, so you haven't shown us the code,
but the problem could be there.