Straight from the FSM:
"Since fuel pressure at the injector is kept constant, the volume of fuel injected is dependent only on the length of time that the injector is energized. The injection time duration is based on engine operating conditions, which are provided to the ECU by the
input sensors."
You need to check your MAP, CPS, CTS, MAT, and TPS. Since the problem you are having is engine speed related I would check the CPS first and then your injector/fuel pressure regulator. Its definitely not your O2, because all it does is adjust the fuel trim depending on your current A/F ratio.
To check the fuel pressure, the FSM says 17.3 psi, which is to be set at 2000 rpm's. There is an 1/8" NPT allen head plug, both directly above the fuel line or 90 degrees from the line (Towards the driver fender. Recommend taking off the hat and the plate the hat clamps to so ease this process). I recommend buying a small liquid filled gauge to just leave in there.