Immobilizer problems

[strity]

Hi all, I have a MB 115cdi Vito and have a intermittent immobilizer problem. The van always starts after a long period of not running (1 hour+) but if I turn the motor off for less than 1 hour and go to start it, 7 out of 10 times it turns over but wont fire up. I lock and unlock it many times, put my foot on the brake, take the key out and in and turn the key 180' and maybe after 5 minutes or 10 minutes it starts. There is no pattern but the above things I do may or maynot help, I just dont know, I also change to my spare key which does not change anything, please help or advise me what to do.