Thank you James for your comprehensive responses. I have reindexed the archive and ran a back up successfully I'll keep an eye on the NAS and the back up disk - or perhaps also the switch (maybe the network has issues).
My schedule has been 'hanging' for a few days unable to complete. The archive seems fine, but the script just won't run. It doesn't stop stopping when I quit either. Perhaps a restart is what I need? It's done this before, and seems to be fixed for a short while after restarting (I shut down at the end of the week), but after running a backup once it does it again. I run OS 10.5.8 on a single processor PPC Mac 1.8ghz, and I back up to an external 500Gb drive where there is over 300Gb of free space. I back up files from an NAS server connected to the ethernet.