MOSFET always slightly on (with Pulldown) and other strange behaviour
Source an enhancement mode power mosfet. Because it seems like the one you are trying to use is a depletion mode type. (or at least behaves like one in your circuit)

Also you should have a resistor in series in the gate circuit so that there is no stray currents developing between the output of the micro-controller to the mosfet. 1K is typical there.