astra jerking ?

thouht i fixed the jerking problem in my astra but still there...i found a split in a small pipe on the air box,drove it earlier and when driving around 30-40 mph the whole car will start kangooroing looks like i'm a learner driver on my first ever lesson,changed the plugs but still the same,the engine management light does not come on,which i suppose is a little harder to dtect,it ticks over fine,and if on a good run it drives fine when going down a hill in gear you can feel it chugging,my astra is a 1.4 16v ecotec s reg 1999,please somebody help!!!!thanks