Time Machine will not do automatic backups

When I set automatic backups to (ON), it resets itself to (OFF) after a while (several hours or less). In the meantime, it normally calculates the hour of the next backup (which never happens). When the computer is turned on or awakens after sleep, Time Machine does an immediate backup, and then turns the Automatic switch to OFF. Otherwise, manual backups works fine.

Please note that I have no software installed that would interfere with the backup and have tried all suggestions posted in the forum for similar cases. This issue may have started after I moved TM to a new WD 6GB Studio II external disk.

Any help would be appreciated.

Imac 27inch 2.8 GHz OSX Lion 10.7.5