Your AC fluid is probably low. I just fixed this exact symptom. The AC compressor clutch is cutting on and off when it should stay engaged. (Open the hood and check it out...) Get a bottle of R134a and a filler hose with guage for $15 at NAPA. (best deal imo) Once the AC fluid pressure is correct the AC clutch will stay engaged (when the AC or defrost is on) and the surging stops.
