Sorry that's happening to you. Nothing is more infuriating than a CTD at the end of a flight. Luckily, I can't recall a single reported case of CTD's with the Dukes that wasn't a one-time thing, or wasn't eventually attributed to something else. The method that I have chosen for programming my aircraft makes them delightfully resistant to CTD's, unlike aircraft that use a lot of WASM instruments.
From what I've seen in my own coding, I always suspect the GPS unit in the case of CTD's, because the simulator's native flight planning system seems to be one of its least stable aspects. For the sake of testing, I would recommend using another GPS unit on a couple flights to see if the CTD can be attributed to the GTN 750/650.
Feel free to report back here, but I might also recommend contacting Just Flight support. They have a knowledgebase for general simulator issues, while I specialize in the aircraft specific knowledge. They will also have access to an XBOX for testing, if that's needed. Please let me know what you discover!