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.

i assume you meant 0.1mm shim?

by the way, which is a better method/way to retard the timing? adding 0.1mm head shim or using a colder plug? and why?

Yes, I meant 0.1mm shim.

For the issue that I experienced, changing the plug to a colder one (in my case a 7 to 8) didn't help at all, in fact made the situation worse. Increasing the head clearance with the shim did the trick. Now my plugs seem to last forever.

The plug looks fine when it comes out.No peppering on top of piston.