Ignition interference
Discussion
Bit more complicated than that (inevitably) - the recommended plug, the B7ECS has no resistive equivalent; if the car is otherwise standard, no reason not to use a BR7ES but, notionally at least, that's not as mechanically robust (different ground electrode). There are other possibles too if you want to jump on the iridium bandwagon (your car will gain 50hp, paint blemishes will magically self-heal, and you'll suddenly be able to play piano and ride a unicycle whilst reciting Faust - or something) 
New 'standard' resistive leads might sort it without changing plugs (if it has Magnecore of similar fitted, which do cause EMI in my experience); as might resistive extenders (almost impossible to find now). Thing is, placcie bodies cars have much less intrinsic shielding so some resistance in the HT side will be needed to avoid interference problems (gives the ignition circuitry an easier life too but that's another story)
New 'standard' resistive leads might sort it without changing plugs (if it has Magnecore of similar fitted, which do cause EMI in my experience); as might resistive extenders (almost impossible to find now). Thing is, placcie bodies cars have much less intrinsic shielding so some resistance in the HT side will be needed to avoid interference problems (gives the ignition circuitry an easier life too but that's another story)
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