2009 G8 GT rough idle and misfire after raining

[1969GiPper]

My 6.0L G8 will have a very rough idle and misfire when the engine is started. However the condition will only happen if the vehicle has been parked out in the rain. Condition does not happen when driving in the rain. Depending how long it misfires a P0300 random misfire code may set in the Powertrain Control Module (PCM). After the engines runs a while the condition will go away. As I said the condition only happens if the vehicle has been parked in the rain.

Last week while on vacation the condition occured when it was parked on a somewhat sloped driveway (front facing uphill). Note: It had rained overnight while it was parked. The condition continued as I backed down the driveway however it went away as I started to back up (front facing downhill) another sloped driveway. Obviously the rain has something to do with it as it never has happened when dry outside.

To isolate the problem I've tried spraying the engine compartment with the garden hose but nothing happened. Any ideas? For what it's worth the car is completely stock and has about 24,000 miles.