About a year after I had replaced the spark plug wires on my 89 Syncro
because one of the actually broke near the plug, I got the sputtering,
etc., but particularly after it had rained.  New wires were recommended,
but, based on some "list info" I got a $.99 packet of dielectric grease from my FLAPS, smeared it
around the connectors, and the problem disappeared whether rain or
shine.  I'd try this first.   Might be this simple.

Maybe it needs new wires.

On the other hand, it could be one of the well known gremlins that
inhabit these FI systems.

Good luck.

Sam