Did someone else do the install? When it wants that authentication, it needs to be someone with admin rights I believe, so if you're using a normal user account, or if you somehow set a root password different from your account password, that might explain it.
In my case, my personal password DOES work for installations, as my account has admin rights.
I just noticed you said "occasionally" it asks. Does this mean sometimes it doesn't? That may be another indicator, since some programs can be installed just for you, under your home directory, and others may not.