Autothrottle surge on approach?

I have noticed this happens mostly with panels that use default(737) fs2004 panel gauges, but when the autothrottle is set on approach the airspeed/engines will be very erratic, surging up/down, even without configuration change(flaps/gear). Of course once autothrottle is disengaged manual power setting are stable. I know this is probably an old problem but i've searched everywhere and see no solution.:-wave