I have posted on this before -- I do not know why this is the case with the 560 motor. All of my problems over the years with different cars doing this has always led to the plug wires and then mostly to the spark plug boot1

It may be that the location of the wires in relation to the heat of the engine, I do not know. I have also tried to check the condition of the wires by spraying water on them thinking that this would pinpoint a bad wire. No Luck -- The only way that you can be sure to find it is to put the car on a scope.

If you think the wires are original -- just replace them! They are not cheap but they will go bad on this car eventually.

The cost of one fix with a scope will pay for the wire set.

Another interesting thing is at two different dealers -- they also always advise putting injector cleaner in the tank.

I have never replaced an injector on any 560 but they both recomended using slick 50 injector cleaner -- I do not know -- I was always told that injector cleaner in the tank was a waste of money!