Could it be an ignition fault? I’m thinking that burning through plugs could indicate either too much voltage or current. I can recall plug faults in a tractor caused by something similar, we were chewing through sets of points and plugs. This was some time ago now so my recollection isn’t clear.
Another option is that there are residues from contamination in the fuel system or combustion chamber causing bridging across the plug. This could “fry” it if there’s enough material.
Either way I’d look at the plug itself with one of those plug diagnostic pictures on the bench in front of me and see whether it sheds any light.
If you’re looking at the second option then it might be necessary to pull the head off and decoke.
