'05 Cavalier Stalling

I have a 2005 cavalier. Prior to this summer it would stall 3x a summer only on humid days or days after a heavy rain. This year once it his 180000km it has been stalling up to 15 times a day. Sometimes at a stop sign, sometimes while driving. It just quielty stalls-no sputtering. I took it to the dealer but they could not find a problem. The engine light came on once but went off before i could bring it to the dealer. Today I changed the fuel filter and spark plugs and put high end fuel in. But it still stalled. What else can I do.