When this happened to me, I decided that maybe it was just a tune-up. I brought it in an my mechanic found that the head had cracked over one of the pistons and was leaking coolant into the cylinders. It had all the same symptoms. I don't know how hot it is where you live, but it got worse for me. On hot days, it would try to stall out when slowing on the freeway. Then, my mechanic welded the head up and the car runs like a dream.