time machine deleting old backups and filled drive all of a sudden

My time machine backup has been moving along for over a year on my i mac. always having about 3 months worth of backups available. about 2 weeks ago everyday I get the message that a backup is being deleted that used to happen once in a while. well now I am down from about 3 months of back ups to only 5 days of backups. the back up drive is full. I have not added any large amounts of files maybe 200 mb of pictures in the past month.

Any ideas? I have the time machine widget time machine buddy, it says everything is ok and that time machine needs to back up 587 new files every hour.

Is there a way to set time machine not to do hourly backups? is there a preference file that might need fixing.