This exact issue happened to me on my Nova Flash Special 0.21.

In my case, the car ran flawlessly until I stopped it. At the next attempt to start, the plug was found warped/distorted.

I added a 1mm shim and problem NEVER re-occurred.

Also, it wouldn't hurt to double check the health of your bearings. Debris could be destroying your plugs (this is only valide if you find your plugs distorted and/or with missing coil.