Inconsistent Power Delivery on SR20DET

[caall99]

so, when i first swapped my sr into my 240sx there was some dirt in the cas, which i then cleaned out, and it seemed to run like a dream. the exhaust has always had a slightly blat blat sound to it. like a small misfire every cycle or so. it idles perfectly, but sounds like it has high duration cams in it (but i know it doesn't). i changed my spark plugs 300 miles ago and they are a light tan in color. so air/fuel ratio seems good. but i still get black smoke when i free rev the engine.

now on to the main problem. on any given day, the car can feel ridiculously fast and on other days much slower (i run 13 psi with a properly installed MBC). sometimes i can feel the abrupt change in power delivery in the power band. when it feels slow, i can let of the throttle for a second and floor it again, and it becomes faster. There are no abnormal noises, except the ugly exhaust note out of my 3 inch exhaust. it smells super rich too. anyway, let me know what u guys think.

could it be one of the coil packs going and therefore creating a weak or delayed spark in one of the cylinders, and causing unburnt fuel to exit the exhaust (BLACK SMOKE) and sound like a misfire?

btw, my turbo does have like 1 mm or less of shaft play. but spins freely and sounds normal when spooling.