How far off the bottom of the drum is the bottom of the the coal basket?It sounds to me as if you have an intake air flow problem. I run 3 intakes and a 2" exhaust the bottom of the basket is 3" off the bottom. I never had a problem keeping my temp. I catch it on the way up instead of letting it run wild and trying to tame it down.
I'm a Proxy Vegetarian> Cows eat grass & I eat cows.