AIR DATA COMPUTER MALFUNCTION ACCOUNT MOISTURE OR ICING IN PITOT STATIC SYSTEM.

Narrative

AFTER FLYING 3 LEGS OF A 5 LEG TRIP WE EXPERIENCED A PARTIAL LOSS OF OUR #1 AIR DATA COMPUTER. PRIOR TO THE PROB ON LEG #4 WE HAD BEEN FLYING IN HEAVY RAIN AND ALSO HAD QUICK TURNED OUR LAST STOP. I BELIEVE THE PROB WAS CAUSED BY ICE ACCUMULATION IN THE STATIC SYS. AS WE WERE CLBING THROUGH FL330 FOR FL350 THE PLT IN THE LEFT SEAT NOTICED A 200-300' DIFFERENCE IN THE ALTIMETERS. AFTER TURNING OFF THE AUTOPLT AND MANUALLY LEVELING OFF THE ACFT AT FL350 WE SWITCHED THE AUTOPLT TO THE RIGHT SIDE. AFTER LEVELING OFF WE NOTICED THE VSI AND ALTIMETER ON THE LEFT SIDE WERE MOVING UP AND DOWN 50-100'; AND STILL SHOWING US TO BE ABOUT 300' LOW. WE THEN DSNDED TO A LOWER ALT AND HAND FLEW THE ACFT THE LAST 200 MI TO OUR DEST. AFTER DSNDING TO A LOWER ALT THE PROB SEEMED TO CORRECT ITSELF. AFTER SITTING ON THE GND FOR ABOUT 40 MINS (OAT AND 65 DEGS F) AND SEEING THAT ALL AGAIN LOOKED NORMAL; I DECIDED TO FERRY THE ACFT HOME AT A LOWER ALT. THE TRIP HOME WAS UNEVENTFUL AND EVERYTHING WORKED NORMALLY. I DON'T KNOW WHAT COULD BE DONE TO CORRECT THE SITUATION; BUT I DO KNOW THAT A GOOD INSTRUMENT SCAN BY BOTH PLTS WILL HELP REDUCE THE POSSIBILITY OF BUSTING AN ALT IN THIS TYPE SITUATION.