Dear Watchjunkie,
I have also recently purchased a new Spitfire Chrono in August 2012 from the IWC Zurich boutique. When using the fly-back function, the whole watch movement occasionally freezes. If I then gave the watch a solid tap, the watch movement will start again. The problem is, in effect, identical to yours.
I have sent the watch back to IWC to see if they can eliminate this problem. If IWC cannot ensure that the fly-back function never causes the movement to freeze , the watch is, of course, unusable.
I fully understand that these sorts of problems can arise in a new model but I understood that the movement was well tried and tested. (I always avoid a brand new movement designs because I want to be sure that the movement has shown good reliability in the field.)
If the 89365-calibre is a well tried and tested movement, the fact that the same problem has hit two users at nearly the same time period, suggests that IWC needs to look into their quality control procedures.
However, if the fly-back function is a new feature, the repetition of the problem may suggest a basic design problem.
I am rather disappointed that, after some six weeks since I sent the brand new watch back to Switzerland, via IWC UK service center, I have not had any feedback from the factory. If it was my business, I would want to keep a customer like myself informed as to progress together with an estimate of when I might get my watch back. However, Swiss Watch manufacturers, unfortunately, have a well earned reputation for not being very good at keeping customers informed!
PS. While not a collector, I am fortunate to own some other watches, including a Blancpain fly-back chronograph. The movement is a F Piguet/Blancpain 1185 movement designed by M. Edmund Capt, one of the greatest watch engineers of the 20th century, who was also the designer of the Valjeux 7750. The 1185 was the first Swiss design to have a vertical clutch and, as a very high class movement, also includes a beautifully executed column wheel activator of the chronograph. M. Capt, made sure that the fly-back mechanism was very well designed and executed and I have never had any problems in using this function or indeed any other functions of the watch.