p0300 and check engine light for #5 cylinder only
 

I have a 1999 blazer Lt 4.3 Ltr Vortec. I own a snap-on scanner and it shows a misfire on cyl. 5 after engine gets to approximately 210 degrees, some times under load misfire starts in cyl. #6 and/or #4. I have changed the cap and rotor and the plugs. I also ran sea foam injector cleaner through the fuel system. Can an injector be intermittent? Could a plug wire be breaking down under load? When you get on it it will still miss on 5 but not as bad. I have been chasing this for days and I have considerable experience with diagnostics. Will a valve stick after it gets hot? Please help if you have encountered this before, I'm desperate!!!

I am having the same issue with a 2001. I have replaced the plugs, cap, rotor and the distributor. The distributor is in as good as it is going to get so I am very confused at this point myself. Please update if you find a cure. Hopefully someone will post soon!

I had a similar problem about a month ago, I had a wire that was arching badly..

Changed the wires, problem solved...