You may have used too much lit charcoal. I only fill my chimney maybe half full, fill the charcoal basket with unlit, then spread the lit coals on top of the unlit. The temp should start out below 200 then work its way up with all vents open. When it gets close to your desired temperature, shut down the vents, then start opening them a little at a time until it settles in where you want it.
That's how I do it with my Excel 20, and have never had any temerature spikes.
