You really shouldn't have to open a vent. The Prop band may be to wide. The prop band is the range of temps where the fan will go on and off. So if your set point is 250, and the prop band is 30, the fan will go on and off from 220 to 280. If the temp drops below 220 then it will be full on. The BBQ Guru folks recommend a setting of 25, but it is pit dependent.
Also, since the temp is responding when you open the vent, it seems that the fire isn't getting enough air. Where is the Guru fan damper set? Maybe open it a little more?