mechanics stumped

["Greg Woodard" <gwoodard@ndsuext.nodak.edu>]

I really need help with this one.

1987 1.8 4cyl.

My son can start the car in the morning and drive it to school (15 
miles)- no problem.  Start it after school and and drive it to work 
(15 miles)- no problem.  Start it 2 hours later and drive it home- no 
problem.

But if he drives around town, stop and go driving, lets it idle for a 
few minutes.  It will start to miss.  He can limp it home, but it 
takes over 1/8 of a tank of gas (14miles).   You can smell raw gas 
almost running out of the exhaust.  The plugs will be completely 
black.  If we clean the plugs and leave it sit overnight, he can 
drive to school- no problem.  

We have replaced nearly everything with known good (ie borrowed from 
my 1.8).  I'm stumped, my sons stumped and several mechanics are 
stumped.  The car is getting really close to the crusher, would be 
now except that my son really likes the car.

HELP

Greg
gwoodard@ndsuext.nodak.edu