[jackbauer]

Ok a few points to consider. The most important effect when switching fets or igbts is miller capacitance. This is like putting a cap from drain to gate. As the device begins to switch on the supply voltage dips causing voltage difference from gate to drain to increase. Unless you have enough power to "punch through" the miller knee the device will not switch on correctly. Have a look at my most recent post on the diy charger thread for an extreme example.

I have found temperature to be a very reliable indicator of correct driving. If the device is cool its happy. Hot is not

You are using a flyback diode right? Another imporant concern is the connection between the diode and the switch. It should be as heavy and as short as possible.

What I would do right now is to switch to an easier load like say a few car headlamp bulbs to see if your problem is in driving or loading.

hope this helps