I had a similar problem a little while ago. I never was watching the phone during the backup, so I don't know what was happening when it would reboot, but the phone did reboot during the backup process.
The way I fixed it was by mounting /data and /datadata, in addition to /system after CWM started. Once I did that, the backup happened successfully and didn't reboot the phone.
I do have to do this every time I enter CWM to perform a backup or restore.
Hopefully this helps.