What was found
DURING DESCENT FROM 15,000 TO 11,000 IN VERT SPEED WITH -1 300 FPM SELECTED, ON THE LEFT AUTOPILOT, THE AIRCRAFT PUSHED OVER TO -3 000FPM. DISCONNECTED THE AUTOPILOT TO PULL OUT OF THE DIVE AND THERE WAS VERY HEAVY PRESSURE ON THE YOKE. HAD TO TRIM NOSE UP FOR ABOUT 5SEC TO RELIEVE IT. DURING THE NEXT DESCENT FROM 11,000, IT DID THE SAME THING. AFTER BEING LEVEL AT 10,000, WE WERE SLOWING FROM 325KTS TO 250KTS ON THE CENTER AUTOPILOT AND THE AIRCRAFT BEGAN TO DESCEND ON ITS OWN. THE AMBER AUTOPILOT WARNING LIGHT COME ON ALONG WITH THE RED AUTOPILOT EICAS. REPLACED APCS ELEC SERVO VALVE AND ELEVATOR FEEL ACTUATOR, ALL TESTS PASSED OK. REPLACED LT FCC, ALL TESTS PASSED OK.