Cal 69385 idiosyncrasy or fault?

I have a Pilot’s Watch Chronograph 41, and I use the chronograph on a regular basis. Intermittently, the chronograph will not reliably restart after a pause - sometimes I must press the start button two or three times before it will resume. I sent it to the IWC service center in Texas. Though the watch received a complete service there, the fault continues. Is this an idiosyncrasy of the movement, or was a problem missed? I have seen no other problems with it. In the ten days since it returned to me, it has gained only a fraction of a second. 

Welcome to the forum, William. 

Not with this movement, but I have seen this before in mechanical chronographs. I've had this occur with a cal 89361, I believe. I am not sure the cause, it has happened very infrequently. If your watch movement has been completely disassembled and reassembled, and the same problem persists, then there may be an issue with one of the individual components. On the backside, can you observe the column wheel moving consistently as you operate the pushers? Is there any difference in the feel of the pusher when the chronograph engages vs when it fails to engage? 

If you have video documentation of the issue, that may be helpful if you decide to send it in again for further intervention. 

Thank you. I did a little testing based on your reply. The column wheel advances every time the pusher is pressed, even when the chrono fails to restart, and the action of the button feels the same. There is no repeatable pattern to the anomaly - it can occur after the first pause or after the twentieth. I have sent video of the anomaly to IWC and I expect to hear from them within a day or two.

Addendum: I just noticed that the small seconds hand also stops when the anomaly occurs. 

Hello William,

your last statement that the small seconds also stops when the problem occurs gives a clearer insight into the problem.

When the problem occurs, the engagement of the chronograph mechanism stops the entire watch. It is most likely a misalignment of the chronograph clutch mechanism. It will take a watchmaker to fix this problem.

br Cromagnonman

Just to add, my first IWC 3716 Portugieser chronograph with the 69355 movement had the same problem, and came back worse after its second visit to the IWC service center in Dallas in 2021. So at least back then they did not seem completely familiar with the correct adjustment, if that's what this is. From what I could see, the tilting pinion clutch would engage the wrong way (teeth-on-teeth, not teeth-in-valley) with the second wheel, and lock up the movement. Eventually the watch was exchanged, but the new watch occasionally (one in a hundred chronograph starts/restarts) shows the same problem, so maybe it is a design and not an adjustment issue.

According to what I found online, the original 7750 design, which the 69000 caliber is based on, has the same clutch, but I have not seen reports of this problem. Hopefully just a few of the 69000 caliber watches have this lock-up problem, maybe due to misadjustment, as cromagnonman said.