What was found
(CAN) AT CRUISE ALT FL190 WITH MAX ALLOWED SPEED, ALL OF SUDDEN LT & RT MAIN GEAR DOORS STARTED TO, WITHOUT CONTROL, EXTEND & RETRACT, RESULTING IN VIBRATIONS IN A/C, THAT HAPPENED SEVERAL TIMES. REDUCED SPEED TO 210 KTS INDICATED & CONTINUED. ON APPROACH GEAR EXTENDED NORMALLY, BUT AFTER LDG NOSE GEAR DOORS STARTED TO OPEN & CLOSE SEVERAL TIMES. NOSE GEAR LOCK SENSOR HARNESS REPLACED. MLG DOOR CLOSED & UPLOCK SENSORS ADJUSTED. A/C RETURNED TO SERVICE.