When doing a MR a couple of months ago I had good luck using Kayak.com.
I wanted to take advantage of the BOS-LAX bonus miles and needed a positioning flight from FRA to do so. I felt pretty good about my results from Kayak which got me from FRA-BOS via DFW-LAS-LAX with 40 minutes to spare before the first of three BOS-LAX RTs. For my homeward journey I was able to fly BOS-LAX-ORD-FRA.