JM wrote:

> I tested the O2 sensor

The O2 sensor has nothing to do with fast changes like acceleration,
and a 1999 Nissan should have a function to check the duty cycle of the
O2 sensor while the engine runs.

Look at the throttle position sensor, MAP sensor, and fuel delivery
rate and pressures (pump and pressure regulator), and see how the
pressures hold up when the pump is turned off, to check for leakage of
check valves. Don't replace parts unless they test out bad, and don't
use a low-impedance analog meter (that is, one that doesn't need a
battery to measure volts or amps (FET-VOMs are OK though), directly on
an O2 sensor or you could damage it.