you may have a wiring fault, i.e. one diode the wrong way around, or you may need snubbers on your diodes, as even lightly loaded there will be ringing on the diodes causing RFI getting into your control and upsetting it - do you have a photo of your set up?
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also not having a soft start (PWM going from 0 - 40% over 1 second) at power up puts largish pulses in the fets and diodes causing noise issues.
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p.s. you do not have an RC o/p filter, just a C.