What was found
DESCENDING OUT OF CRUISE FROM FL310 TO FL300, TEMP -33*C "AP MISTRIM ND" ANNUNCIATOR ILLUMINATED. ELECTRIC PITCH TRIM WAS FROZEN. AT APPROXIMATELY 12,000 FT., TEMP +1*C ELECTRIC PITCH TRIM FREED UP, A/P ENGAGED, AND CONTROL OF THE AIRCRAFT WAS NORMAL. LANDING WAS UNEVENTFUL. MAINTENANCE REPLACED THE LEFT AND RIGHT PITCH TRIM ACTUATORS TO KNOWN FLEET TREND. A SUCCESSFUL OPERATIONAL CHECK FLIGHT WAS PERFORMED. AIRCRAFT WAS RETURNED TO SERVICE.