Service Engine Soon?

I have a 94 grand prix and I bought with a new engine with about 89k mi. (It was a replaced motor). I get up past 60, start to drop speed, usually about 50-55 My SES light comes on. I turn car off and its gone, same thing happens. Oxygen Sensor maybe? Also my Radiator Fans dont kick till bout 250. Is that 20 degrees a big deal? Thanks!