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Hi electrowizard,
looks like you have an interesting problem, im an electronic engineer so hopefully i can help.
It might be easier to help you out if you post pics of schematics and any waveforms that your able to get.
As jackbauer mentioned capacitance is an important consideration because if it is not overcome quickly by driving the gate with high current the fet will stay in its active region for longer (where it drops both voltage and current at the same time) which will create heat. So if your device isn't getting hot when it is working of the second or 2 then you have no issue with this capacitance. I think someone suggested the semiconductor could be getting hot but not the case, this can not happen, the semiconductor will be bonded thermally to the metal tab so that heat can come out of the device. General rule of thumb if you can keep your finger on the FET its not getting too hot.
Its definately worthwhile testing with just a resistive load rather than a motor to test out your circuit. So try a light bulb or a resistor of more that 1 ohm. If your FET still dies with a very light load then you are driving it incorrectly.
Note: you are trying to drive an inductive load ( motor) so you might be getting voltage spikes feeding into your system which could be more than hundreds of volts unless a suitable diode is placed to disipate this.
Also make sure you dont have the mosfet backwards drain where the source should be and source where the drain should be.
Do you have adequate heat sink, if your starting a motor with this the heat dissipated will be large, tracks on a PCB wont be enough, probably need a chunk of metal
worth checking you have the correct parts, check the part number on the device. I have had digikey send me parts incorrectly although they are generally very good mistakes do happen.
If you have access to a oscilliscope probe the gate, drive and source pin, check for any high voltage spikes. A multimeter on a maximum hold setting wont cut it.
If your low load test works and you still dont know what is wrong start increasing the load progressively until something pops
Good luck
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