1993 mercury topaz engine light

[momo734]

ok i dont know if this has been answered already but ........ here it goes ...
i got a 1993 mercury topaz gs V4 ..... and when i get around a steady 40-45 mph my service engine soon light will come on and stay on for a little while ..... there is really no specific time when it turns back off but most of the time its when im slowing down .... i have gotten a new thermostat put in and a new radiator under the advice of my mechanic , hoping it would do the trick but it didnt ..... he has put it on the diagnostic tester and put it through every test he can think of .....ive asked him about the possibility of replacing the o2 sensor under the advice of a friend and he shot that idea down ...... so if anyone has any idea what this could be please let me know .
i would very much appreciate it.......