starting problem with my 2004 nissan almera?

2004 nissan almera.. on occasion's car will not start does not matter what type of weather, it ain't the battery as i replaced it, eventually after anything between 30 seconds and 20 minutes it will start, could it be the immobiliser as sometimes the key light on dashboard will come on... can anybody put me in the correct direction to sort this very annoying problem out.