@Jules V. next time you use the lid, close one set of the vents closed and see if that make any difference?
Your total lid vent surface is about 9.6in2 (based on 7/8" diameter vent hole), by closing one set, it will bring open vents surface area on your lid closer to the bowel vents surface area which is about 6.2 in2.
By having a larger vent opening than the bowel vent opening you might be loosing some heat.