Operations Engineering Bulletin No. 164/1A refers to this occurence reported recently (Sept '04)
Transient speed target drop, in cruise, after a F-Plan modification.
After performing a DIR TO, the managed speed target drops below the current airspeed, and the autothrust commands idle to follow the speed target change.
There follow's a lengthy explanation, which those of you in posession of FCOM vol3 may like to read.
Moley.