A faulty O2 sensor can cause problems but I don't remember reports of one affecting performance without throwing a code. You may be able to monitor output with your OBD tool; normal operating values should be in the FSM (link in my sig).
Check your Intake Air Temperature (IAT). The ECM limits power when it gets over about 90 degrees F or so. If temp is OK, I'd try cleaning the MAFs, as per Z_ealot.
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