Only runs roughat 40-50 MPH??

Hi We just helped my son buy his first vehicle a 1994 Grand prix SE. We bought this from my wifes aunt so I know it has been taken care of and all service has been done by a dealer. The only problem the car has is at 40 to 50 mph the car runs really rough but as soon as it is above or below 40 to 50 mph then it is fine. What could be the problem??
Thanks for the help!
George

check your plug wires and make sure they are not arcing, had this happen to me with my old grand prix, it was a 94 also, if that isnt it, it may need to be scoped

could be EGR valve