Help needed in uk -sparkplug Gap closed

[geoff11]

Hi ,
Just found this site and have a question, I have a MG ZT which uses a mustang 4.6 mustang engine as standard, trere were approx 800 of these made in uk its an 04 2 valve, anyway just over a year ago Adrenaline sport and racing in UK fitted a kenne belle supercharger made a massive difference in power from 265 to 410 hp drives so smooth , but theres an underlying problem, about a year ago whilst in top on motorway at about 80-90 mph, whilst flooring it it statred misfiring, limped bach nome, found one of the spark plugs the gap had completley closed up ? anyway, dealer changed it and all was well , until about a month ago playing with an M3 on motorway at about 90 same thing,
this time 2 cylinders had there plug gap closed right up and another cylinder had a coil pack go but the plug was fine , all was well until only a couple of days later on the motorway up a hill doing almost 90 ( again) and it went all spluttery, limped home
and decided to get spannering my self , one of the plugs had closed up as before , luckily the dealer handed me the remainder of the old good plugs,
once fitted car was back to normal.
Do any of you good people know whats causing this , and should the company who did it for me sort it for free ? I paid about £ 6000.00
so not cheap .
any feedback welcome, theres very few people over here who know anything about these cars
Plugs are TR6

Thanks Geoff Wakefield