VandykeT/A

My heads/cam car has a part throttle stumble occasionally between 2k-3k with around half throttle under load. And by stumble I mean a very light almost not noticeable stumble. At wot the car pulls fine just at part throttle. I also noticed on my wideband the car will run really reach like drop down in the 10's-11's at part throttle under load. Also for instance if I just start the car, let it idle say 10-20 seconds and then leave the car will spit and sputter until it warms up and again will show extremely rich on the wideband, 10s-11s. At wot high rpms it'll usually peak 11.9-12.1 which again is rich but not as bad as the part throttle and cold. The car was tuned about a year ago and I don't really remember it doing this then but then again I'm very seldom in the car. The only changes to the car from then and now are I went to a 4" exhaust from my 3" catback and added a CHRS1313. I would think if anything the car would be lean rather than pig rich after the mods. I'm thinking I may have a bad coil. I recently changed the plugs in the car and just before I pulled them I started the car, let it idle 5-10 seconds and pulled it up about 10 feet and killed it. I noticed the number 8 plug was black but the other were a white brownish color. Whats the odds the number 8 coil is a little weak?