I have the 4.0 thor disco 2, I managed to get it running on all 8 for a week or so then suddenly the other morning, it was down again on one cylinder, after faffing about with plugs and leads finally found the problem, cylinder 6, is not firing, so i take the plug out, (fit spare plug to hole) and fire her up, the removed plug wont fire when touched to the manifold but will fire when held away from any metal or earth, Weird, so i plug the ht lead onto the spare plug thats in the head,(old ford escort plug or something) and it runs sweet on all eight, Then it gets weirder, If i have the old escort plug in the head and the pfr6n-11 in the ht lead and touch it to the escort plug, the engine runs sweet... So it runs okay on the old escort plug, will not run okay on the original plug but will run okay when the original plug is touched to the old plug but not the other way round (escort plug in ht lead and pfr6n in head) its weird and also the no spark when touched to manifold but full spark when held away from everything........... HELP....... Feel like burning the fooker.!

new lead and plug-local motor factors
If the plug is bpr6es then you don't want a resistive lead with it

Found the problem, The plug had been fouled due to a weak spark from a very badly corroded ht lead, Broken inside the rubber part, cleaned plugs and new ht lead set and its running fine again. FOR NOW>!!