B737-800 SUFFERS UNRELIABLE FO PITOT STATIC INDICATIONS IN ICING CONDITIONS AT CRUISE. DESCEND OUT OF RVSM AND EVENTUALLY REGAIN CORRECT INDICATIONS.

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FLT TO ZZZ W OF ABC VOR CRUISING AT FL360. FO'S AIRSPD STARTED TO DECREASE; FOLLOWED BY 'UNRELIABLE AIRSPD' INDICATION BELOW AIRSPD INDICATOR. THEN 'ALT DISAGREE' INDICATION BELOW ALTIMETER. CAPT'S ALTIMETER READ FL357. FO'S ALTIMETER REMAINED AT FL360. WE XCHKED WITH HUD AND DETERMINED THAT CAPT'S ALTIMETER AND AIRSPD INDICATOR WERE READING CORRECTLY. WE REQUESTED LOWER TO EXIT RVSM AIRSPACE. WE WERE INITIALLY CLRED TO FL330. UPON LEVELING AT FL330; HAND FLYING; THE ACFT WAS DIFFICULT TO LEVEL OFF. WE REQUESTED LOWER AND RECEIVED CLRNC FOR LOWER WHEN WE WERE SLIGHTLY BELOW FL330 AND THEN GIVEN CLRNC TO FL280. WE XFERRED CTLS TO THE CAPT AND SELECTED AUTOPLT 'A' SINCE THE CAPT'S INSTS WERE GIVING CORRECT INFO; THEN PROCEEDED TO COMPLETE THE QRH ITEMS FOR 'UNRELIABLE AIRSPD; ALT DISAGREEMENT; AND EEC ALTERNATE MODE.' CONFERRED WITH DISPATCH AND MAINT AND CONTINUED FLT TO ZZZ AT FL280 WITHOUT FURTHER INCIDENT. FO'S AIRSPDS AND ALTIMETER SLOWLY RESUMED INDICATING CORRECT INFO AFTER EXITING IMC. THIS EVENT; WE THINK; WAS CAUSED BY ICE ACCUMULATING ON THE PITOT PROBES ON THE FO SIDE. SINCE PROBE HEAT WAS ON AT THE TIME; THIS INCIDENT SHOULD NOT HAVE OCCURRED.