Recurrent dental issues / root canals

I am a 33 yr old healthy female with good dental hygiene.  I have been plagued by recurrent deep cavities and have required root canals on seven teeth in the past four years.  For the last one, my dentist said he had never seen a tooth decay so quickly (looked fine in a x-ray 6 months prior to requiring root canal).  Two of the root canals have not "taken" and required retreats (and I feel a third coming on).  

I have seen a number of dentists and endodontists and they're all treating the individual cases instead of identifying the cause of them.  I am using prescription tooth paste and trying to avoid dry mouth at night in case that is the cause.  I am getting a little desperate and need whatever advice is out there.  Does anyone have any idea why this is happening to me?