distressed 84 4000S

[Hiok-Tiaq Ng <htng@aries.ece.cmu.edu>]

My friend has an '84 4000S which has been experiencing difficulties.  
Occasionally while cruising, and/or on startup, rich-black smoke pours
out the exhaust and the car almost (occasionally) dies.  There is no
reqularity to this problem, sometimes it occurs twice a week, sometimes 
just twice a month. BUT, it has been going on for over 2 years and no 
dealer has been able to figure it out!!  The car's maintenance schedule has
been followed to a tee and it's a very well kept southern car.

Any ideas??

Thanks,
Tiaq
htng@aries.ece.cmu.edu