When spark KV gets this high on average, 96 Windstar 3.8V6 78kmiles original plugs, it can be as simple as dirty injectors, here is my procedure for injector flushing to see if it is a factor.

This is the only thing I use for flushing, a basic pressurized bottle with gauges and shutoff valve, I use a quick disconnect main line and several adaptor lines to fit various rails, most are your basic schrader port.

This is the chemical I use for all makes, I started using this after I'd seen gm's tsb for cleaning injectors and I have been doing it on all makes with great success. One that I recall, mid 90's Ford pickup 4.9 6cyl that would barely idle cleaned up and idled so smooth I was amazed.

This gets mixed in a 10% solution with gasoline and is pressurized at the rail to 25psi, vacuum to pressure regulator is unhooked and plugged, and of course fuel pump is disabled, and start and run for 15 to 20 minutes. Some makes require more pressure to stay running or the injectors are so plugged more pressure is needed, but I have never gone past 30psi on the rail. The important thing with this mixture, is that is does not go back to the tank, if too much gets in, it may damage the pump, so if you get much past 25psi, pinch off, or unhook and plug tank return line.

Here is the spark KV after the flush, a definite drop in kV, idle smoothed out, and power was back without doing anything but the flushing. The plugs in this were in need of replacement and were done after the flush which is common practice after flushing, but I have found this procedure does not make a mess of the plugs after flushing, so if plugs are good, they need not be changed after flushing.