weird plug fault

[chuggernought]

i have a disco, 3.9, standard lucas efi auto with lpg(induction type), k&n filter, flame thrower coil and magnacor leads.
i've have always used ngk plugs on all my v8's(and i have many) for some reason no1 plug always cracks the ceramic around the electrode after a couple of months and naturally induces a miss fire, all other plugs are ok its always just no 1. the plug looks fine with the perfect slight tan colour.
had bosch and champion plugs in the past and had nothing but trouble, ngk have always been perfect till now.
any ideas?
i'm guessing this has been an ongoing problem for some time before i bought the motor as the plug threads are badly butchered, next time it needs changing i'm going to have to have the head off and helicoil, perhaps that will reveal a reason...

[ChrisJC]

Any evidence of water getting onto the ceramic?

In my experience, the Rover coil & ignition amp are perfectly OK on LPG provided the leads are good.

Chris.