i had a renault clio 1999 v reg & it had the same prob, i fixed it myself this is how to do it stage by stage
1 = turn the key in the ignition until all the lights on the dashboard light up but DON'T START THE CAR
2 = open the bonnet and disconnect the battery but leave the keys in the same position
3 = wait 20-30 minutes then reconnect the battery, you should here a sound like an alarm dont worry about this
4 = now try and start the car this should bypass the immobiliser this worked for me & i never had any more probs after that good luck
paul fulton, April 2011