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
'08 Weber Genesis Gasser
SJG Tamale Pot Mini Smoker (SJG was a rescue)
and a Pyro FIL