1998 toyota camry stalls at idle

My camry stalls often but not consistently when stopped at a light or stop sign. There is no change in behavior
wheather the engine is cold or warm . If I keep the rpms at about 1 k it will never stall and runs smoothly.  While
driving the engine runs totally smooth and never misses, even at high and low speeds. The symptoms seem to
arrive somewhat suddenly. There is no check engine light and there are no odbc error or pending error codes. I
have read all the posts on this subject and none seem to fit or help.  I have changed the engine coolant temp
sensor, the engine idle control valve, cleaned the egr valve, pulled the vacuum hose on the egr valve, replaced the
air cleaner filter. I would appreciate any advice.