What was found
PIT - FLT 1764 - EN ROUTE FROM PIT TO PWM DURING CRUISE AT FL 370, THE CAPTAIN'S ALTIMETER, AIRSPEED INDICATOR AND VSI BECAME INOPERABLE. NO CHANGE OCCURRED WITH CLIMB AND DESCENT FOR 15 MINUTES. FLIGHT RETURNED TO FIELD AT PIT AND WHILE ON DESCENT INTO PIT, INDICATORS BEGAN OPERATING INTERMITTENTLY. ON GROUND, INDICATORS OPERATED NORMAL. NO EMERGENCY WAS DECLARED. MAINTENANCE REMOVED AND REPLACED THE NR 1 AIR DATA COMPUTER. OPERATIONAL CHECK OF AIR DATA COMPUTER AND ASSOCIATED INDICATORS WAS NORMAL. (X)