Need to verify some things. When the engine starts popping, does it still run well? Or does it get jerky or tempermental? Does it ever pop when you first start it up? Or only after it's fully warmed up? Have you ever driven it continuously for more than fifteen minutes without stopping? Did it start popping while you were driving? Or only after you stopped and then restarted the engine?
The distributor might be getting worn out, but if it is, you'd notice erratic behavior because the timing would change randomly as you drive. So it might be running well one minute and then poorly the next.
The popping sound - is it metallic? Hollow? Deep sounding? High pitched? Is it rhythmic? Random? Stuttering?
