I am not positive that it was the ocillation that destroyed them. I have not investigated the electronics further. I also think the frequency of my PWM's may have been too high. They were running at 400hz. The fet's may have overheated due to the high inrush each time the PWM cycled. The faster you run the fet's the more they heat. I probably should change the resistor that controls timing and slow one down to around 15 to 20 hz and try again. I just haven't had time and do not want to burn another one out. I will change out the fet's to bigger ones on one of the bad PWM's, see it it still works, slow it down and try again.