This problem is often a symptom of dropouts that are out of alignment. The inside dropout faces should be parallel with each other. A good shop should have the tool to check this. You can make your own with all thread, some nuts and some washers. If you decide to make your own, PM me and I'll show you how. This could also be the result of a loose skewer. The skewer lever should be set so that it begins to tighten at the 90deg position in it's 180deg throw. But check dropout alignment for sure. If this is an alignment prob, have a shop do the work to avoid voiding any warranties. Contact Cannondale if you run into resistance at the shop.