Is the top vent open all of the way ??
You may also want to close the bottom vents about half way and let you smoker come up to temp slowly. Filling the charcoal ring up should not restrict airflow at all, you will also want to make sure that the ring is centered in the bottom so as not to restrict the air flow.
Water in the pan will also help control the temps. I will also recommend that when lighting your chimney that you only have about 12-15 coals in there, this will allow better temp control as well.
I hope that helps.
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