If you're letting it get to 300 it'll take a bit of time to get it back down.
As said, try lighting less coals to start with. Let the drum come up to temp SLOWLY and shut down the intake when it's about 5 degrees from your target temp. If I want mine to run at 225 I have to almost close off my intake valve completely, or at least BARELY crack it open.
You'll figure it out.
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