What was found
AIRCRAFT WAS GROUNDED: PILOT REPORTS WHILE DESCENDING ON AUTOPILOT (2) IN IAS MODE SET TO 290KTS, THE AIRCRAFT BEGAN TO DESCEND UNUSUALLY FAST IN VERTICAL SPEED WHILE MAINTAIN 290 KTS. THE AIRSPEED ON BOTH F/O AND CAPT'S INDICATIORS THEN QUICKLY DECREASED TO 230 KTS. APPROXIMATELY 10 SECONDS AFTER INDICATING 230 KTS BOTH CAPT AND F/O'S AIRSPEED INCREASED TO 350KTS, BELIEVED TO BE THE ACTUAL AIRSPEED. ALL PITOT HEATERS INDICATED GOOD CURRENT DRAW. NO FLAGS. STANDBY AIRSPEED APPEARED TO BE CORRECT. MAINTENANCE REPLACED THE #2 AIR DATA COMPUTER IAW AMM 34-16-01. RETURN TO SERVICE TEST NORMAL FOR BOTH DFGC IAW AMM 22-01-05.