Pinging

Check for a knock sensor on the side of the block, (driver's side, as I recall). If you watch the timing light, you should see it retard momentarily when you tap the sensor with a small hammer. If the timing doesn't change, replace the knock sensor.

I can't remember if there is a computer module on your car. If there is not, there will be a vacuum advance mechanism on the side of the distributor. Use a hand vacuum pump on that servo to see if the timing changes when you increase the vacuum. If it does not, check the mechanism to see if it is leaking or sticking.