problem out of the litter box

My cat, 11 years old, recently started having bowel movements outside the
littler box. This started about 2 months ago and happened every 5/6 days. In
the last weel she has done this 4 days in a row. She has been to the vet
numerous times and he doesn't think it is a medical problem. He suggested an
extra litter box, uncovering the one we already have and even possibly
changing the litter......all things we have tried....still she goes where she
wants. It's odd that she occasionally uses it. I keep it very clean (as the
vet suggested), but still can't seem to get this under control. Any
suggetsions?????????????? I really would apptreciate it!