OS 10.5 - Time Machine cannot mount disk after "Sleep"

I have setup Time Machine to work with a ReadyNAS. It works fine after start up. Time Machine finds the network drive and backup is good. The backups continue as long as the computer is in use. The problem starts after the computer "SLEEPS". After that I keep getting an error message from Time Machine "Backup volume cannot be mounted". I can still see and access the backup volume through Finder.

When I go to Time Machine Preferences it shows the backup disk under Change Disk, but I am prompted for password to the disk after I pick it.

Been fighting this for some time. Any suggestions??

If this is the wrong forum - sorry.

Thanks is advance.