What the OP is referring to is generally known as "limp mode". It's a base setting which is used in the event that a critical problem is detected. Often it's not really something big, an unplugged sensor, a leak,
Check for error codes, check the TPS' alignment, inspect all of the intake plumbing for holes/cracks/or other leaks, make sure the AFM harness plug is connected firmly to the AFM, check the knock sensors and their wiring.
A bad O2 sensor may cause you to run a bit rich or a bit lean but anything too far out of range and the ECU stops reading the O2 sensor altogether and reverts to the stock fueling maps... Also, at WOT, the ECU doesn't even read the O2 sensor; so, if you've got problems at WOT it is definitely something else.
One other possibility is that the timing belt skipped a tooth or wasn't properly installed causing uneven tension which when the motor is finally turned over puts the pulleys out of sync.
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