Time machine image corrupts every week or so.

I have a new Apple Mac Pro, 10.7.3. This is backing up to a NAS.

The first backup will work and complete a backup. This will work for about a week, backing up successfully. Then it displays the message:

"Time Machine completed a verification of your backups. To improve reliability, Time Machine must create a new backup for you."

"Click Start New Backup to create a new backup. This will remove your existing backup history. This could take several hours. Click Back Up Later to be reminded tomorrow. Time Machine won’t perform backups during this time."

I cannot get rid of this message without starting a new image.

However this is a pain. My 300GB of data takes more than a day over WiFi to create new image, and I loose all my history. I author documents, and do actually want to keep my history.

I have two other Apples backing up successfully, they do not sugger this problem.

Can any user help me find out what is wrong with this computer?

Many thanks!

Ben