Autothrottle stops responding on approach

Over the last two days I've experienced a problem which I have not been aware of before. Firstly my understanding is as follows. If flying with the autothrottle engaged VNAV or the MCP Speed/N1 will control my speed. Even if I disengage VNAV or the autopilot the autothrottle will continue to control speed whilst armed. I have been quite happy wih this logic for some time, however yesterday and today I did flights into MMTJ ( TIJUANA ) and had problems.Basically the approach to runway 27 is a VOR/DME one.In this scenario I tend to hand fly as soon as I can see the runway. As the approach has some terrain the decsent is a little steeper than standard (3.5 degrees). I dial in the apprach ref speed of say 145 and when appropriate kill the autopilot. On both occassions the speed started dropping well below the commanded speed. On the first occassion I only noticed when it was too late and crashed. This morning I was waiting for it and had to kill the autothrottle and apply full power but did manage to get down in one piece.As I've been flying the NG for a long time now I'm wondering if this is an 800/900 feature or have I managed to ovelook some fundamental point during all previuous flights.