While intercepting the localizer; a B737-700 experienced uncommanded pitch and autothrottle anomalies. The event was apparently related to the inappropriate capture of the glideslope from below and a short lived low airspeed deviation ensued.

Narrative

Observation was from the jumpseat on an approach to Runway 10R during night time IMC. The pilot flying was cleared to descend to 2;500 from 3;500; given a vector to intercept the final approach course and cleared for the approach. During the right turn to intercept at approximately 2;800 FT; the aircraft pitched up to approximately 15 degrees nose high and the airspeed rapidly decreased to the boxed airspeed of approximately 115 KTS. The pilot flying asked the pilot monitoring to select Flaps 15 just prior to this occurrence as he was slowing down and configuring the aircraft. The pilot flying had the autothrottles engaged using the level change function to descend from 3;500 to 2;500. The throttles were flight idle and airspeed was Flaps 5 airspeed on the bug. The pilot flying was using the heading select feature for the intercept and I believe he selected the APP mode as well. It is my belief the autopilot captured the glideslope and was trying to initiate a climb to intercept; and the auto throttles added power for the climb but not quickly enough and thus the rapid loss of airspeed. If this is actually what occurred; then it needs to be made known how the aircraft will respond with autothrottles engaged and an intercept below glideslope.