Hi,
I had exactly the same experience over an extended period of time. However, as mine is a weekend toy only, it never has much exposure to rain. I did, however, find that over time the ignition coils (around the leads), and/or ignition switch units were burning out (especially after the engine was exposed to a high operating temp for any length of time) leading to the same loss of power, and stalling problems.
Switching off and re-starting would clear the problem, with the length of subsequent smooth running seamingly corresponding to the amount of time I'd left the engine switched off (to cool down / dry out, it seemed).
The bad news for me was the problem went unsolved over a period of 5 years, despite services, and expensive component replacements, at several W. London Porsche specialists.
The good news came after I decided to try a service of the ignition system at a Bosch specialist, as opposed to a Porsche one, and made the discovery that No 5 Garage in Acton, London specialise in both. They tested the car using the Bosch diagnostic system (more sophisticated than a 'Bosch Hammer' download from the car), and decided that one of the HT leads was incorrectly routed, leading to ignition shorts over a period of time, and especially when the car ran in the rain for any length of time. No new parts were needed, and the charge for labour was less than £100.
To be honest, I haven't driven the car in heavy rain for any length of time since, but the car has run faultlessly for the last 18 months, including after periods when the engine has run hot (including extended runs to Le Mans, and on a week-long roadtrip with my kids across Northern France). It also seems to pull more strongly, and run more smoothly, than it was before the service. So, I'm pretty sure No 5 Garage have correctly identified and solved the problem with my car.
Hope this helps.
