Sometimes after I would drive it to a store & cut it off, when I went to crank, it would run very rough blowing black smoke.  After a few minutes of gently pumping the pedal, it would run smooth.  Of late it does not matter if it is cold or been on the road for a half hour, suddenly it will loose power & start the skipping mess w/ the black smoke.  The injectors are new & everything adjusted.  Gauges on the fuel pump says things are fine.  The afm was replaced & the second time I drove it problem was back.  The guy working on it was the main Z guy years ago at the dealership.  He is clueless.