The Ghibli has a pre-mix power burner. That means it has a fan on the burner that injects air and gas in different proportions relative to the desired heat level. This means your roaster exhaust and burner are sometimes interacting with each other in highly unexpected ways.
Many roaster manufacturers use this system (usually from Weishaupt or Honeywell) as a it's a pre-packaged, off the shelf, drop in UL/CSA internationally certified solution. Sadly, it kind of s-cks when you are trying to precisely control the application of heat when roasting speciality coffee.
Eventually, you'll get enough experience on the roaster to figure out optimal settings (batch size, preheat temp, airflow, drum speed, and heat level) that minimize the need for changes in those setting throughout the roast. It's helpful sometimes to simply turn the burner off and on during the roast. Perhaps preheating higher and "coasting" through the first minute or turning off the burner coming into first crack and "coasting" through a period of development time.
The Ghibli is a well designed and well constructed roaster. You can absolutely coax good coffee out of it. It just requires that you be a little more creative to manipulate your roast profile. Or shift your graph up and to the right into the darker roast territory it's more specifically designed for.
