MOSFET PWM switch jamming on - why?
When using paralleled MOSFETS in the output stage as you are, your should install small ballast resistors (0.2 Ohm) on each source terminal to ground. This will ensure that the MOSFETs share the current equally. 

...The way this works is if one MOSFET starts taking more of the current, this will lead to a lowering of the Vgs of that MOSFET which turns the MOSFET off slightly and re-balances the current between MOSFETs.

Also, its best to give each MOSFET its own independent gate resistor, this can eliminate any potential oscillations (at least thats recommended in amplifier circuits with paralleled output transistors).