External DLL freezes editor, not build

I have an external unmanaged DLL being called from a C# script using DllImport. The strange behavior is: it works the first time I push the play button. Then when I stop the playback and restart, the Unity editor freezes and I have to end task it. However, if I build the Unity project into an exe, it works fine no matter what. Even stranger is that I've used the DLL in a MonoDevelop C# project (independent of Unity) and it works perfectly fine. What is going on here? If Unity is running on Mono, and the DLL works with Mono, why won't it work more than once in the Unity editor?