IIRC (but it's been a while since I looked), the FSM has a pretty good description of how fuel trim is supposed to work. Might give you some clues as to what's going on.
My guess is that the ECM is seeing some spikes from an O2 sensor. Eg, there is a post on this site about someone getting extreme readings when an O2 sensor wasn't fully warmed up. If you can't find it using the site's search, try one of the major search engines and add "site:the370z.com" to your search string.
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