Um, pardon me for the obvious question................
You wouldn't happen to be running Mepis live from CD, would you? With no permanent storage for it to save to between runs?
My only other guess is that something re-writes fstab on shutdown/startup. That sounds like trouble! Can anybody be accessing your machine who removes the line you add?
I run Mepis strictly as a live distro, so I wouldn't be any help with creating persistent files and making them initialize on boot.