I have read somewhere that fire control is done by running with your exhaust vent wide open and controlling the air flow by adjusting the intake vent. I'm almost 100% positive it was here on the Brethren. I had problems with my offset and temp spikes, but once I learned to control the air flow, the spikes became minimal. I'm wondering if by closing the top vent most of the way that it is causing the heat to be trapped in the cooker.
Red Valley BBQ Team Head Honcho ~ Vice Pres. In Charge of Production Firefighting BBQ Team ~ KCBS CBJ