2 Answers

It would seem that an internally shorted coil (windings) would do all the things you are describing. Worn or excessively gapped plugs would make your coil operate at a hotter temperature on an already hot engine operating environment. I was jumping the battery of a friends 2000 Focus at night and saw spark jumping through the distributor cap close to the holddown screw shorting out power to that corresponding cylinder. Spark worn cap terminals and/or rotor might have caused that burnthrough causing a hot coil operating atmosphere also. Months before I had replaced the plug wires that a mechanic had supposedly changed, I saw spark jumping through the insulation so I pulled hard on the wire in that area and saw it stretch. It turned out the stretchable area had a 1 " green span where the wire arc burned the core of the wire. If you find that changing the coil fixes the problem, check all the things mentioned above. I always apply silicone dielectric grease to both ends of all new wires I install or old wires I  successfully remove without damaging.

There might be a fault on the output voltage from the alternater,to the transfomers feeding the coil,