Problems starting

my car had been off the road for nearly two years. due to this i changed the battery. by doing this the key has gone out of sync with the immobiliser. I managed to retrieve a code which I need to enter every time I need to start the car.
However the problem is sometimes the car will not start giving me a sense of insecurity when going out. Before putting it back on the road I did have the starter motor changed. Witnin a week car would'nt start. I had new starter changed again, only this time when changed and all well the following day it would'nt start. You never guess what.... changed starter again, worked for a week then not starting. at present it is starting. Sorry for the long winded senario. tunny boy