Vauxhall Vectra 1999 2L Pet. Engine trouble?

Anyone any idea how to fix my Engine Management System, its a bit tempremental and the light will come on start up (not every time though, normally 1 in every 3 starts). It restricts engine performance and also has a tendancy to stall the car whilst sat in traffic in first gear. Anyone got any clue as to what might be causing this?