I am assuming that the bumping is caused by superheating the liquid passing though the heat exchanger and it then flashes when in enters the low pressure of the evaporator. I think it might help to set a limit to the temperature difference between the heating media and the material being heated. Increased circulation rate through the heat exchanger or increased agitation in the evaporator kettle, if it is jacked heated, could also help. In both cases, (lowering the temperature difference or increasing the circulation rare), this should help to keep the liquid at the heating surface from reaching a high superheated state that creates a large volume of flashed vapor all at one time when the pressure is reduced.