After pulling the gas tank for clear access, a light smear of dielectric grease on all the electrical junctions* and also the fuse connections may help. I have the plug wire boots wire tied into the coils as well. That one is a particularly weak connection, and the wires are known to fall out on occasion. Perhaps yours are taking on water at the coil side? Just a guess. If all that doesn't help, some new plug wires may be in order. Also - as part of a pre-trip inspection, I found my plug wires green & cheesy at the coil side. I believe I had too much air gap between the two contacts.
You might also want to check the carb rubbers and airbox, to see if water is making it's way in there as well.
Some forum members have reported hard starting when running in the rain. Just for good luck, I often idle for a few minutes to dry things off before shut down, when running in the rain.
*(I wouldn't put any dielectric grease on the high voltage spark plug or coil connections)
