1999 Cavalier stalls when running at low speed

[Cacao]

Hi,

I own a 1999, 2.2L, automatic trans. Sometimes it stalls when running at low speed ( 20 mph or less). It does it periodically, other times it runs perfectly, no noises, temperature is always ok. I've done the computarized engine control several times at different service centers but no problems are found, the only thing is that the O2 sensor data screen shows almost no voltage variation always below 1. I’ve change the camshaft and crafshaft sensors but seems I’ve lost my money. Any idea? Please help!!!