I've had the same exact problem. Everytime it'd rain hard, or even a really heavy dew would make it sound like a Volkswagon. I had two problems... the distributor was getting condensation inside the cap, interrupting the electronics. I also had the 6 wires in the harness that plugs into the module come loose, and they were getting wet too. If I could get the engine running long enough to dry it out under the hood, it was ok. Otherwise the car had to sit for a day or two. Drove me nuts till it was found.