96 GMC 1500 5.7 vortec

[js54705]

Hi Im new to this have worked on cars and trucks along time but for whatever reason I cant seem to figure this out. I have a 96 GMC 1500 5.7 VORTEC runs great when first started go and drive it for about 20 mins starts to misfires only when parked and at idle Have done full tune up replace spider injectors mass airflow sensor map sensor i get a p0300 code but when check misfire data on scanner its number 4 that seems to be the problem if i let the truck cool for awhile truck runs fine till warmed up again also bad sulfer smell out tail pipe any help would be great