P0300 along with a blinking SES light is almost ALWAYS caused by faults in the secondary ignition system: Spark plugs, plug wires, distributor cap, distributor rotor, ignition coil, and ignition module. These engines are very fussy when it comes to replacing them so don't cut corners trying to save money. Start with a set of AC Delco plugs, and an AC Delco distributor cap, and rotor. Put a dab of silicone dielectric grease on each terminal of the cap, inside and out. Use Autolite Professional Series wires, they're cheaper and better than AC Delco wires
Next step is check fuel pressure and leakdown. Key ON, engine OFF, fuel pump running: pressure must be 60psi to 66psi and must remain above 55psi for 3 to 5 minutes after the pump shuts off. Post your results.