97 Tracer funny stalling problem

My friend owns a 97 Mercury Tracer....she has had it for just about a year, bought it off of her older cousin. She says that the car stalls out when the car first starts up and runs for about a minute. I was told it only happens sometimes and the weather conditions cause the car to run very very rough. We're are currently trying to figure out what the problem is. I told her to put fuel inj. cleaning in her tank and she did so, and the problem consists. On hot or sunny days the car runs great but when it is rains the car runs really rough and tend to stall out. We have driven the car and the only problem we can find is that the car might need brake pads....other than that nothing. Any info would be very helpful, thanks.