I have a 1995 425 with a Kawasaki 20 hp water cooled engine w/650 hrs. It runs fine for about 30 to 40 minutes then the left cylinder loses spark. I ran an inline spark checker to verify the spark loss. I assumed the coil was going bad or wire was going bad (they come as a unit). I replaced the coil and plug cap and still have the same problem although the spark is brighter. I cleaned the connections at the main circuit board and checked the fuses; all OK. The temperature guage always reads normal.
Once it loses spark it sometimes regains the spark for awhile but it eventually loses it. If I let it sit for an hour it usually has the spark when I restart it but after 30 or 40 minutes of cutting it stops.