I think I am going to start with the downstream 02 sensor. I had it checked for exhaust leaks and all seemed fine. The mechanic took a rubber mallet and rapped on the cat and no rattles could be heard.
My gas mileage is 29 mpg with a mixture of highway and around town driving. She seems to be running fine, not miss firing.
So, I'll start with the 02 sensor..... we shall see ;-)
Thanks for your quick response! Very much appreciated!