I'm suspicious of the coil wire. The energy stored in the coil will take the easiest path to ground when the power transistor fires it. If there's too much resistance, it'll jump somewhere else - like to the frame of the coil bracket as engine ground. It still could be the coil itself breaking down with heat, but wires are usually cheaper than the OE coil assembly. Try a junkyard for the coil assembly and/or a good set of plug wires.

I'd recommend the Magnecor's if I knew why they don't fit right, but maybe I will after I take Ray's suggestion and call them. A little squeeze with a pair of pliers works, but I hate to have to do that with something that expensive which should work better out of the box. They are good wires, though.

Frank