2008 Ford Focus Cold start problems and stalling

Hi, my husband bought a used 2008 Ford Focus in 2012 (I believe) that was in great condition. We started having problems in January 2015 when it was exceptionally cold (below zero, we live in NH). My husband couldn't get the engine to turn over when trying to start it first thing in the morning. The battery was fine and it cranked ok, it just wouldn't turn over. Hours later once the temperature was higher (around 10 degrees) the car turned over on the first attempt without any other issues. This happened several times over the next couple of months but only when it was below zero. In February, there were times when the car started without any issues but then stalled after my husband was driving it for just a couple of minutes. He was not able to get it going again until later that day. We took it to the Ford Dealer and they were not able to see any code issues. They cleaned the fuel injectors and replaced the fuel and air filters. My husband also added dry gas a couple of times previously and even switched to a higher grade fuel. The problem still exists even though are outside temperature is warming up. Yesterday the car started on but stalled down the road even though it was 21 degrees. We couldn't get it going again and had it towed to the Ford dealer. They called us later that afternoon stating they couldn't find the problem. They kept it overnight to supposedly check the fuel pump but today they are still saying they couldn't find anything wrong. It cost us over $400 the first time and no charge this time but the problem isn't resolved.
If you have any suggestions that would help us we really would appreciate it. Thank you so much!

Debbie and John