Hi James,
At very low load conditions, the LM2611 will be in pulse-skipping (hysteretic) mode since the energy stored in the Cuk capacitor is more than the energy required by the load. The Cfff will have no effect on the control loop, so this value will not make a difference in such low currents. You can decrease the values of the resistor divider network (RFB1 and RFB2) by a factor of 10x (so that RFB1 = 2.94K and RFB2 = 1K) and increase Cff by the same factor (3300pF) to provide the same loop response when back at full load. Alternatively, can your application tolerate the use of a permanent load RL resistor to pull some extra load current?
Best Regards,
-Juan