In my experience, those engines are picky about spark strength. Generally, I suggest using only AC Delco plugs and wires in GMs. At that many miles, and twenty years old, if the wires have never been changed, they are not right. They can look fine, but still not be delivering a full strength spark.
Also, at twenty years and 140,000 miles, it would not be a bad idea to replace the fuel filter. https://www.2carpros.com/articles/how-to-change-a-fuel-filter
Finally, take the cap and rotor off and clean off any corrosion and make sure there is no moisture. The design of that distributor can allow condensation to buildup inside the distributor, causing rough running and/or misfiring. I take the cap and rotor off of my Blazer occasionally (same design, except for a six cylinder engine instead of a V8) and clean off the contacts and make sure it is dry. That alone has eliminated a P0300 code before.
Wednesday, September 27th, 2017 AT 7:33 PM