I finally decided that the solution to my growing list of problems was to upgrade, and have done a new and clean install of LL 2.4 on my laptop.
My first problem is that Dropbox is not working, for reasons I haven't experienced before. Nothing seems to be happening and I keep getting an 'authentication is needed to run /usr/bin/dropbox' as the super user message, a step I have never seen before and also appeared the first time I ran Skype on the new install. I hope this is something that won't have to be done every run-time, but for Dropbox, nothing satisfies it - my authentication just causes the error message to reappear some seconds later.
Is something misconfigured or wrong with the new install?