I have mine on a watch winder because I'm too lazy to adjust all the indicators. However, my other Valjoux 7750 don't need to use the chrono start button to begin working... Neither I needed to use that chrono start button when I received the watch and started using it.
Another test you may do is to gently swing the watch, so the crown is moving from left to right, for about 5 minutes. Every full swing (left to right) should have a one second duration. That should also fully charge the power reserve and you may check for how long the watch will work.
Have you made contact with Maurice Lacroix or some service center near you?
Anyway, by reading your comments, it seems that you're normally wearing the watch and it's not stopping anymore, so maybe your initial issue is being slowly solving out by itself.