The 1998-2003 Lexus RX300 may start and stall, or start and run at very low engine speeds when the engine is cold. 

The issue is a failed or dirty idle air control valve. This valve is mounted under the throttle body, and modulates airflow to the engine under certain conditions. When the valve becomes dirty, or when the valve fails, proper airflow to the engine is not achieved. This results in start and stall, and low RPM.  

To correct the issues, the idle air control valve will need to be tested, and cleaned or replaced. 

To help decrease the frequency of this issue, change the air filter no less than once per year.