I had the same issue with an old SCCA car I ran years ago. An old mechanic friend of mine came by one night when I was in the garage playing with the car and I told him what I was dealing with. He opened the hood, shut all the lights off, fired up the car and revved the engine to about 4k RPM. Looked like a lightning storm under the hood with the sparks flying from one wire to the others and the block. He told me that even one wire, under load, can have a pinhole fault and cause all sorts of carp.
Long winded answer - sorry. I'd change the plugs and wires - and make sure that the plugs are the correct ones for the engine - not what the parts store cross-references. (Found that out the hard way, too