I have found with my evaporator in order to have consistant draws I need to try and keep the fire as consistant as possible. Big batches drawing off are a sign that the flue pan is taking a big slug of sap. Also when drawing off I only open the draw off valve to a trickle, my draw off temp will rise a couple of degrees but nothing too bad. I also found last year to avoid a big draw after starting the evaporator I take off about 2 gallons of sweet from the previous boil and adding that into the draw off box. This helps avoid the ripple effect of big draws. Remember if you need to add defoamer to the syrup pan only add it to the partiotion nearest to the side you are drawing from. Good luck