Time machine does complete backup every 2 - 3 weeks. Why?

Every 2 - 3 weeks TM starts on a full backup. This results in my 1TB backup running out of space too quickly and needing to remove earlier backups. With exclusions I am only backing up abouit 130GB and do not make many or large changes from day-to-day.
Sometimes I can fix the problem by doing the reset proposed by Pondini (http://pondini.org/TM/A4.html) but not always.

Can anyone PLEASE offer any suggestions, as it is very, VERY Annoying to have go through this procedure time and time again.

Regards,

Dave

PS: my previous posts on the topic are also relevant.