how can i fix my vauxhall corsa 1.4 16v sport?

for a while now my engine management light comes on within the first few miles of a journey,it then goes off and on randomly through out my journey. in slow traffic or pulling away from stand still is where i have problems, it dies out when i try and accelerate away. it then takes a sevral attempts to get it started again as when it turns over it doesn't hold it revs and dies out again. eventually when it does start the revs are erratic this problem always occurs when i've been driving for 20 mins or more.
i have no problem from a cold start.