I had the same situation in my 360. Turned out to be the first symptoms of the cats going bad. That said, it could be anything form that to a bad sensor.
Any codes other that non- ready?
Thanks for info, Killer58. Have already replaced cats. We changed some of the sensors, as well. A friend suggested that maybe we need to unplug the "numerous" sensor connections and clean with contact cleaner, we'll probably do that next.'96 F355 Spider 18k miles, recent engine out service. The ECU will not go ready on all codes, even after many tries at recommended driving procedures to set the system.