Medium Large Transport; Low Wing; 2 Turbojet Eng
Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine; Flight Crew Instrument
Confusion; Human-Machine Interface
ATC Issue All Types; Ground Event / Encounter Ground Equipment Issue
Flight Crew Overrode Automation
ATC Equipment / Nav Facility / Buildings; Airport; Procedure
While on approach to Runway 30 and flying the ILS 30 approach the aircraft automation intercepted the glide slope from below after a momentary fluctuation from the glide slope signal. For a split second the glide slope indicator fluctuated from full up to full down about 15 miles away from the airport. At that moment the aircraft automation captured the glide slope from below and the aircraft started an abrupt pitch up attitude. To fly the glide path and at that moment I disconnected the autopilot and continued the approach manually (there was no deviation from ATC instructions). The glide slope indicator did that a second time right before landing again along with the flight director. It took us a bit to understand why that happened; but here is my theory.When landing west in Miami; the airport uses Runway 30 for landing and 27 for takeoff (intersecting runways). Every time an aircraft was cleared to take off Runway 27 the moment that aircraft passed adjacent to the glide slope antenna there was a fluctuation with the glide slope indicator in the aircraft; and with the flight director since they were coupled. We flew this approach many times and on different aircraft since then and every time the same thing happens. When an aircraft is cleared for takeoff from Runway 27 and passes the glide slope antenna we received a fluctuation (full pitch up and down from the flight director and glide slope signal). My recommendation would be for the FAA to investigate and see if this is true. Also for the time being I would recommend the airport uses the RNAV approach to Runway 30; or publish an amendment to the approach to not allow an autopilot coupled approach. Thankfully we caught this situation early when we got the abrupt pitch up attitude of the autopilot following the flight director and glide slope signal; but it can easily turn into an unusual attitude situation if caught off guard.
Air carrier pilot reported experiencing a malfunctioning glide slope signal during approach to MIA.  The pilot disengaged the autopilot and landed uneventfully.