check engine light

I have a 1990 gmc 1/2 ton pick up with 305. the check engine light keeps coming on but not right away and not all the time. it usually comes on after about 20min when i first start the truck. but if i turned off the truck and turned it back on it turns off. i checked it with the little plug thing and the code that comes up is with the exhaust related issue. i chaged my sensors and that didnt work. please help with any other suggestions

my 88 does the same thing, i replaced a fer sensors and it still does it. but when the light is out it sitll passes smog, so im not too worried about it. actually when i wen tto smog it the light came on and went out when i got to the place, passed with flying colors

Mine only comes on when i floor it. i have a 90 with a new 350. It goes off wen i turn my truck off too... Maybee a exsaust related issue? Maybe cuz I have headers? More flow? Screw up the sensor?