What was found
SEA - FLIGHT 2319, AT 24,000 FT GOT A IAS MISMATCH. FOLLOWED CHECKLIST. SWITCHED TO NR 2 ADC. ON DESCENT NR 2 AIRSPEED WAS DECREASING. THE STANDBY AIRSPEED INCREASING WITH ELEVATOR FEEL, PITCH TRIM, SPOILER OUTBOARD, AND RUD CONTROL, CAUTION LIGHTS ILLUMINATED WITH AUTO PILOT AND YAW DAMPER FAIL. SWITCHED TO NR 1 ADC AND AIRSPEED RETURNED TO NORMAL AND THE CAUTION LIGHTS WENT OUT. CONTINUED WITHOUT FURTHER PROBLEMS. REMOVED AND REPLACED NR 1 ADU. OPERATIONAL CHECK GOOD. AIRCRAFT RETURNED TO SERVICE.