Immobilizer - works some time- doesn't work some time?

I have reanault clio 2 1.2. Since 3 days my immobilizer doesn't function properly. Initially it started with the problem that i have to do on and off the ignition for a few times then it works. Now withing 3 days it started taking more numbers of trys. Infact yesterday it took almost 1000 try then suddenly it works. One interesting thing i noticed is that if i drive for a while then switch off the engine and try it immediatly, it is having no problems at all. Problem starts only if the car is off for some time.

Can any one suggest what could be the possible solution.