Sounds like you've got some kind of intake air flow issue. I've never run all my intakes open for more than start-up. All I need is intakes closed and ball valve at about 3/4 open to get steady 225-230. I'm using a Weber lid which may make a diference.
2004 Weber Performer GREEN
Orange Super-Duper Fast
Taylor Made 9deg. R7TP w/S-Fujikura 6
Rescued 2002 Weber Genesis Silver B Gasser
SJG Tamale Pot Mini Smoker (SJG was a rescue)
18" Gold (rescue) SJS (rescue)
and a Pyro FIL