Why does my Vauxhall Vectra cutout?

W reg Vectra, 1.8litre, Starts at first touch, always, but when starting from cold will stop if revs are reduced down to tickover. After a mile of driving, engine tickover is no problem. However I think revs are set too low, 500rpm, and once engine is warm revs will vary, 450-550rpm every few seconds. Is it a question of raising revs? And how do I do that?