master pages not stable

[obair81 at comcast.net]

For the first time ever, I am finding that frame is, on its own, changing the application of master pages. 

About once a day, I find that frame assigns the "first" master page to a page which most definitely did have "left/right" assigned to it.

If I re-assign the correct master page to the 5 or so "mis-assigned" pages in this 60-page manual, I find that in a day or so, the master pages are changed again. 

This does not happen when I save and close the file and frame, and re-open. It appears to happen in the course of adding text and the manual page count growing.

This is frame 7.2.

Thanks for any help.

best,
Paul