I had the same issue. The assigned ringtones would work properly for a little after re-booting the phone, but would not work properly a shorttime after. This went on for a long time and I finally went to a Verizon store where the tech told me that he was aware of this problem. He flashed my phone and after a couple of months, it still rings properly.
By the way, I have my ringtones grouped into the media/audio/ringtones file format like others.
By the way, I have my ringtones grouped into the media/audio/ringtones file format like others.