Fred - just for kicks, try using less charcoal in your 18 incher. Use just enough to get a nice even layer across the charcoal grate instead of a full chimney.
I noticed using my Smokey Joe that the temps weren't getting as high as on my performer and I finally figured out I was using too much charcoal, which seems to have been inhibiting airflow and slowing down the charcoal combustion.
I cut back on the charcoal and the little guy will hold 500 degrees plus for over an hour.
Pat