Mysterious Loss in Traffic

I have a website that contains about 250 pages of content that ranks very well in the SERPS. We were averaging around 10,000 uniques per day during March and April. At the end of April I began to have major disruptions with the server host which resulted in the site being offline on and off for several weeks. I decided it was time to change web hosts. I moved the site to a windows dedicated server. But something happened when the DNS was changes as we have lost almost half of our uniques each day. We are currently running around 5,000 uniques. Some of this loss can be attributed to seasonality, but a huge portion has fallen through the cracks some where. There were no changes made to any of the page code, either before or after the move so it is not likely to be "on page" issues. I need some help diagnosing what happened to all of our traffic. This project will have two phases: Phase one: Researching the issue and writing a report as to what happened and what should be done to correct it. Phase two: Implementation of your recommendations. A bonus of $.01 for each unique over 5,000 per day for thirty days after the conclusion of the project.