What was found
AT APPROX 34,500' WHILE IN IMC CONDITIONS, EXPERIENCED UNRELIABLE AIRSPEED INDS ON BOTH PRIMARY & STANDBY SOURCES. FLUCTUATIONS (APPROX. 250 KIAS-100 KIAS) LASTED FOR APPROX 8 MINS, DURING WHICH TIME THE AUTOPILOT & YAW DAMPER DISCONNECTED & ACFT BEGAN TO DUTCH ROLL. CONCURRENTLY, OBSERVED AN UNCOMMANDED THRUST REDUCTION BY AUTOTHROTTLE SYS AS WELL AS SPLIT THROTTLE POSITION DURING UNCOMMANDED THRUST REDUCTION. NO AIRFRAME ICING OBSERVED DURING EVENT & SHIP'S ONBOARD ICE DETECTORS DID NOT INDICATE AIRFRAME ICING. ONCE CLEAR OF IMC CONDITIONS AIRSPEEDS RETURNED TO NORMAL & ALL AUTOMATED FLT CONTROL SYS SUCCESSFULLY ENGAGED. POST FLT MAINT INSP REVEALED NO ANOMALIES OR DEFECTS WITH REGARD TO THE PITOT STATIC SYS & NO MOISTURE OBSERVED IN THE SYS. FOLLOWED BY A FULL PITOT STATIC TEST WITH NO DEFECTS. POST FLT ANALYSIS BY MANUFACTURER INDICATES THIS EVENT LIKELY DUE TO INADVERTENT FLT INTO ICE CRYSTALS, WHICH ARE UNDETECTABLE TO AIRCREWS.