One cause for off idle stumble on warm-up is when the ECU runs in 'rapid catalyst heating mode'. This is only activated when coolant temperature at engine start is between 10C and 35C, the ECU retards timing to dump heat out the exhaust to get the catalytic converters up to their spark point sooner. This mode is disengaged above 55C and when RPM > 2600, something else to consider during your troubleshooting.