P0300 series DTC's are almost always caused by secondary ignition faults: distributor cap, distributor rotor, spark plug wires, spark plugs, and ignition coil. The 4.3 is quite fussy when it comes to these parts. Stay away from aftermarket stuff and use AC Delco, especially the plugs, cap and rotor. At 100,000 miles all of these items should be replaced, along with the oxygen sensors, air and fuel filters. It's a good idea to check fuel pressure and leakdown also... anything over 100,000 miles on the fuel pump is a bonus. Again, AC Delco, don't waste your time or money on anything else.