engine problems

when i drive my jimmy.. 87  after driving for about 10 min's the check engin light will start to flash when i come to a stop. If i switch to neutral or park while im like at a red light the light will go off for the time being. Does anyone have any idea what the problem is?
 
 

i believe that the flashing means a misfire.

so ... what should i do to fix it.. how come if i put it in park or N it stops.. if i ride long enough the light will just come on and stay on.