My old dentist did several fillings a year ago. In June I saw him again and he said that the decay continued under one of the fillings and he re-filled it. Recently I had to have a root canal on that tooth because the decay kept going under the second filling into the root. I also have another tooth that was filled and needed to be filled again. I am now seeing a new dentist. My mom went to the old dentist and told him what happened. He said something about the type of decay that I have being "insidious" and a certain variety that keeps going even after the cavity has been filled. This doesn't make sense to me, because you would think that once the tooth was drilled and all the decay was gone then the decay couldn't continue because there is no decay left... Has anybody heard of this type of decay before? Any advice short of resigning myself to a constant stream of root canals?

Sounds to me that the 1st dentist didn't clean out the decay completely on those 2 fillings before he placed the new fillilngs. You may need a base put in the tooth before the filling is placed.
See how the new dentist does. Glad you left the other. Look up the word insiduous in the dictionary, it will give you a definition of the decay that he "says" that you have.
It appears that he's making an excuse for leaving a small amount of decay under the new filling.
Not all dentists are bad, but I worked for some that needed to generate more production and would think about doing something like that especially when you're on an HMO and they aren't making enough $$ doing a simple filling.

Good luck!