i would check to see if the o2 sensors have gone bad they will throw you a check engine light when they go out
I have the same problem. It is the transmission.
I have a 1999 Chevy Blazer and shortly after the Service Engine Soon light came on, my transmission started "slipping" between 1st and 2nd on an automatic. This only happens when the car gets warm on trips longer then about 15 miles.