Immobiliser problem

[Kylie]

Hi, I have had a bit of trouble with starting the car, I changed keys to see if it were the above. It starts first pop when I use the other key, but now wont start with the one I usually use. The starter motor was replaced last year and everything else is running great. I have heard the ignition swtich can get loose and needs securing? Has anyone else had this happen and what did you do to fix it for good!

97 A4 Avant SE