Most of the talk on the Focus forums seem to indicate that the problem is your speed control sensor. [http://www.theautopartsshop.com/Search/Default.aspx?Pat=ford+focus+vehicle-speed-sensor&Key=auto+car-usa+parts&gpartnum=03ForFocus/DESS10702&utm_source=google&utm_medium=camp3&utm_content=Vehicle%20Speed%20Sensor&utm_campaign=ford|You can find one here, for not that much money.]
Good Luck,
Frank