Engine light

I own a Rav4 L 2003 version. Its a great car and I love it but recently I started having problem with the car. I took it to a different place for oil change and two days after getting the oil change my check engine light came on. i took it to VIP and they told me that there were some cylinder misfires and suggested me to change spark plugs. I did it and again after two days the check engine light came again. I took it back to VIP for diagnostic. They told me that it shows cylinder misfire again but said that they could not find anything wrong with the car. I decided to take it to toyota dealer. The same thing was told to me by the guys at Toyota workshop, there is cylinder misfire and they could not find anything wrong with any of car, as through the diagnostic. They turned the check engine light off. The guys at Toyota service centre told me that I should let the car warm up before driving. I took their suggestions but after couple of days the light came again. The one problem that I have with my Rav4 is that when i start the engine it races for a while and it takes like 5 minutes for the race to come to normal position. I took it to several places and some technicians also told me that it is common with RAV4. Anybody has any suggestion if it is normal for RAV4 to race like that and why i am having cylinder misfire? I live in Bangor, Maine (extremely cold weather situtation).