A CLBING S80 IS UNABLE TO MAINTAIN ITS ASSIGNED ALT WITH ANTI-ICE SYS ON. AN ALT EXCURSION OCCURS WHEN THE ACFT IS FELT TO BE IN AN INITIAL BUFFET.

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THE EVENT STARTED WHEN WE ENCOUNTERED LIGHT TURB AT FL310. I WAS THE PF. OUR WT; 123000 LBS; ALLOWED A CLB TO FL350 SO WE ASKED FOR AND RECEIVED CLRNC TO CLB TO FL350. WE WERE IN CLOUDS AND THE ANTI-ICE SYS WERE ON. CLB PERFORMANCE SEEMED POOR AND I ALLOWED THE AIRSPD TO SLOW TO .64 MACH WHICH WAS ABOVE THE MINIMUM INDICATED ON THE PMS. AS WE APCHED FL350 I FELT A PRONOUNCED BUFFETING WHICH FELT LIKE A LOW SPD WING BUFFET AND THE WING DIPPED SEVERAL TIMES AS IF IT WERE NEAR STALL. THERE WERE NO OTHER INDICATIONS OF A NEAR STALL CONDITION; IE; SPD LOW; ETC. I IMMEDIATELY ASKED THE FO TO REQUEST LOWER AND STARTED DSNDING. WE RECEIVED CLRNC TO FL310 AS WE APCHED FL340. THE CTLR DID NOT COMMENT ON OUR ALT. GIVEN THE CIRCUMSTANCES I WOULD HAVE DECLARED AN EMER IF WE HAD NOT RECEIVED CLRNC FOR LOWER AT FIRST CONTACT WITH CTR. ALL INDICATIONS I HAD SAID THE ACFT WAS CAPABLE OF FL350 AT THAT WT AND AIRSPD. I DID NOT DO ANY ABRUPT MANEUVERING DURING THE EVENT BUT SOME PAX COMMENTED ON THE BUFFETING. SUPPLEMENTAL INFO FROM ACN 418259: THERE WAS NO INDICATION OF ICING OR ANY OTHER CLUE WHY THE PLANE WOULD NOT HOLD FL350.