A B737-800; cleared for the ILS approach from 5;000 MSL to Runway 28R at ORD; suffered a severe pitch over and loss of altitude when the autopilot captured an apparently false GS as it descended rapidly from the top to the bottom of the 'cage.' After recovering from the event; the aircraft following them on the approach reported an apparently identical anomaly. Following the second event the Local Controller stated words to the effect of '...the heavy's out of the way now'.

Narrative

When cleared for the ILS to Runway 28 we were cleared to maintain 5;000 FT. The First Officer (pilot flying) selected [armed] the Apch Mode on the MCP [Mode Control Panel] as we were approaching a fairly large rain shaft on final. Suddenly the glideslope indicator came off the upper peg and swept very rapidly downward and the FMS grabbed it and the airplane pitched over violently as the glideslope indicator went full scale from top to bottom. By the time the autopilot was disconnected the pitch change was such that the airplane lost almost 400 FT and was recovered by about 4;600 FT. Almost immediately another aircraft behind us reported the same thing happening to them. The First Officer recovered quickly and climbed back to 5;000 FT. I don't know if the glideslope signal was faulty or if there was interference from another aircraft on the approach or the thunderstorm on final. Regardless; the First Officer did a nice job recovering and getting back on the approach and landed in a very strong wind with no further incident.