try limiting Vgs to 10V, using a voltage divider. with the 10k resistor you already have, a 4.7 k resistor going from the gate to the switch should suffice. the 4.7k resistor and the gate capacitance will also serve to debounce the switch (to debounce it better add a 1 nF or 10nF cap across the 10k resistor). there are noise spikes every time you operate the switch, which the FET amplifies, and the burst of noise could be reflected by the load back into the FET. what is your load? is it a motor or other highly inductive device? if so, you might get an inductive spike that could kill the FET. yes there is a body diode in the FET, but that doesn't automatically protect the gate from being sent outside the +/-20Volt Vgs limit. when you check the D-S "junction" of a MOSFET, the gate should always be connected to the source during the test. also you said the device was heatsinked, which in the photo, it's not...