first post here bear with me..

just bought the truck not too long ago, thought the missing was just wires-plugs-cap-rotor, replaced all and bearly helped (was pretty bad off in all gear ranges and rpms) later took to a mechanic to have it timed since i didnt want to mess with a distributor in the back of an engine, he tells me that it is fixed position and cannot be adjusted (is this true??) today started reading up in the forums deciding i needed to do it myself and learn more about the pickup, and learned that the EGR valve is an easy check and possable problem, cleaned it up and it got rid of almost all of the problems at idle and thru 2nd gear (woot). however 3rd still strong but deffinately misfireng/stuttering 4th gets worse and 5th is almost useless...

mechanics code read O2 sensor, he reset it and now all i get is random misfire 5th cyl

what should i start checking next? preferably stuff i can do without putting money into it unless its pinpointed the problem

btw its a '96 z71 1500 5.7L vortec fuel injected 150k mi but i dont know much beyond that

-edit- just found out about the ac-delco spark plugs being recomended, gonna replace the junk ones i bought with that tomorrow