Mine used to do the same as your describing i did a full tune up and sea foam the engine and sea foam in the gas tank and replaced the cat. Ran good after that but try tune up and sea foam to clean out the gunk in the engine

 

how do you sea foam the engine? i seen a video once where a guy pour it slowly in the brake vac line while it was running, is that right?

 

Tune up first.

http://www.seafoamsales.com/how-to-u...treatment.html

 

Cut one open and you'll see why. The secondary windings have very little mechanical protection from the segmented iron core they wrap around. A little heat, a little vibration and the insulation becomes damaged...followed by internal arcing.
Operating one with a plug wire disconnected is often enough to kill it, the insulation is so deficient the additional voltage punches right through.