I was thinking about that secondary air system also as it feeds air into the system. I would certainly check carefully all those associated vacuum hoses and solenoids; the EGR system is attached to the intake manifold as well.
The PCM uses the difference between the front and aft O2 sensors to set fuel trim. Front sensor is primary, aft sensor mostly shows catalytic converter function.
The MAP sensor is also used to set fuel trim in the NA.
Might want to check the contact points on the rely in the RCM for the fuel pump; bad points could be reducing voltage to pump, but I would think that would show at all RPMs.