Hi Barry.
I was looking your schematic and if I may suggest, replacing Q1, Q2, R16 and R17 with a Mosfet Driver chip, like microchip's, would give you much better drive to the mosfet Q3.

If you have a scope, look at node of the two emitters. If it shows that the two transistors are both conducting for a short time you may be waisting energy and heat the drivers. Also the edge that drives the Mosfet may not be very steep.

Ioannis