Wasn't there a Britannia 757 that got a bit slow during an early level-off on a SID a few years back? I think they had such a high rate of climb that the ALT CAP mode engaged several hundred feet before the pre-selected altitude and somehow the thrust reduced to the level-flight value required for the current speed even though they were still climbing for several seconds. As a result the speed reduced to an uncomfortably low value. I seem to remember the CAA or AAIB investigated.
My memory is not what it was so all the above may be wide of the mark. Anybody else remember this incident?