#1 Thank you all, & my apologies, it's the TO220 that I'm using, loosing, it was late when I posted #1. The lost FET got so hot that the surface of the tab no longer looked smooth and uniform and was freely movable.
#2 a larger heat sink may be in order but would not have saved my FET, IMHO.
#3 Direct contact with heat sink not feasible as there are 4 FETs on this heat sink that form an H Bridge.
I think the problem is that the FET gets hot, the nylon softens, the contact pressure lessens, poorer heat transfer, FET gets hotter, nylon softens more and on and on. If the nylon didn't melt, the FET temp would rise, but the temp would level out within the operational range of the FET, thus my switch from nicely formed nylon insulators to the tedious use of shrink tube and fiberglass board.
Thank for the confirmation of my thoughts about the mica insulator.