Whats UP?2000 1.3 l Echo sluggish engine

I have a 2000 model 1.3 ltr Echo which runs like a dream - except that its very sluggish in accelerating from a still position. It drives well otherwise and is very economical - but it was recently refused registration by the licensing auhtority on the basis that the engine and driveline didnt run correctly under acceleration. It was recently given a full service so I am fuming that it didnt pass registration. I changed the spark plugs yesterday but it is still sluggish when accelerating. Does anyone know what could be wrong?