What was found
IAS COMPARATOR CAUTIONS ON PFD'S LOSS OF 10 KTS ON CAPTAINS SIDE WITH GAIN OF 15KTS. FIRST OFFICERS IAS LOST AIRSPEED. BOTH CAPTAIN & FIRST OFFICER AIRSPEED BEGAN TO DECREASE. ACFT WAS IMC AT FL 370, SAT -55. ACFT ANTI-ICING WAS SELECTED TO ALL. AIRSPEED INDICATIONS CONTINUED TO DECREASE. CLEANED LT & RT PITOT MAST MOISTURE DRAINS & DRAINED WATER FROM P1 & P2 LOW POINT DRAINS. NR 1 & 2 ADC'S OPERATIONS CHECKED OK IAW AMM CH 34-15-00.