92 Taurus Bucks/surging.

Hi Guys, New to this forums. My wife 92 GL as develope a bucking, surging problem at cruising speeds. It idles and accelerate fine but when you cruise at 55 or 65 it begin to surge. I scanned for code with a star tester and it passed. I am thinking vaccum leak, but I can't fine anything. Thanks for any input. Pat