There are a couple of possibilities. One is that the place you buy gas (if you always buy from the same place) has some problem with their 91 octane tank and you are getting cruddy fuel, watered fuel, whatever. You might tru burning through an entire tank and buying somewhere else (Texacos are generally pretty trustworthy).

There is also the possibility that something is wrong with the vehicle, although I can't remember enough about the difference cotane ratings make and how it might interact with certain engine conditions. A friend who is far more knowledgable (car nut and a geologist who works for a large energy concern) tried to explain it once, but I can't remember the specifics.

good luck,
rt