Just trying to make a point... I was going through the recommendations of National Fire Protection Association. It says you should replace it in two definite cases:
1. If it is over 10 years old.
2. if it is working erratically.
In your case pete_c, the 2nd scenario holds good. So, per my opinion, replacement will be the key for you. However, I also agree with Dards1975 as that should be the first option.