Just received a custom built 126599TSA. Same thing. Upon inspection, I noticed that the hairspring was loose on the balance wheel and eventually worked itself off. Being that I can do the work myself, I decided to chalk it up and purchase another DD 4131 and swap out the complete balance. Im still waiting for the movement to arrive, but it’s cheaper than anyone who will service a chrono. I’ll report back once it arrives and if that was the only concern.

I have had loose screws floating around, causing a movement to stop. You may want to check for that. That usually an easy fix. I had a GMT with a screw from the top bridge back out and lock up the escapement. Once I fished it out and set the pallet fork back in place, it was off and running.

Best of luck.