If the problem is absolutely confined to the scenario where you are rolling to a stop, with no additional symptoms otherwise, I would start with the clutch. I've seen it more than once in the Cambiocorsa and the QP where for different reasons, the clutch remained in contact with the flywheel a fraction of a second too long, which triggered an engine shutdown when rolling to a stop. The NCR is programmed to throw the car into N during such an event but a total disengagement is required to move the gearbox into that gear and if something prevents it, then you'll see this symptom when rolling to a stop. Flywheel glazing, clutches not properly broken in, or low clutch slave pressure are some of the issues that will cause this symptom by itself. I serviced a 2007 QP experiencing the same issue due to leakage in the engagement solenoids. Not saying that's what is wrong with yours - it's just something a F1 tech will look at should it not be anything simple and obvious.
It's impossible to diagnose such a vague condition over the internet, so this is just my guess, based on the fact that you said it only occurred when "while I roll when I slow down." If the car has a rough idle when warm, delayed start, or anything else then I'd say it's fuel/air or some engine management issue.
The only way to be sure is to have a tech look at it during failure, preferably with the SD connected, that way the hydraulic pressure can be tested along with engine management. Otherwise, we're all just spitballing.
Erik Di Somma
1989 228 (past)
1985 Quattroporte III (past)