This may be a stab in the dark, but how old is the internal backup battery?
Good thought. I don't know the age. I purchased in 2009 but it was a refurbished machine which, according to the serial number, was from early 2008. I don't know if 'refurbished' means a new backup battery.
Quick side question: Following the system install & archive, I am now about 5 GB larger...which is causing problems for my 80GB Time Machine backup. i.e. it can't.
EDIT: It was my fault. I hadn't emptied the download folder of all the OS updates. Duh!
Do you know how I locate of anything archived so I can trash it?