I have the exact same problem on my 94 A4. It just started hapening about two weeks ago, and has gotten worse. yesturday it rained really hard, and after getting off of work it would not start. I had to let it sit until this afternoon to get it to start. I had to hold the gas pedal to the floor while cranking (clear flood mode) until it finally tried to start and stumble. I let off and cranked again and it started, but on only about 5-6 cyl's. I revved it to 2000 rpm and held it there until this missfiring stopped, I could feel and hear each time one of the cyl's started firing again until it finally was firing on all 8. I drove it home (36 miles) with absolutely no issues. Like yours, mine runs great when it is dry.
I did not have time to diagnose it today, but will be working on it tommorow. I'm a mercedes tech, and diagnose driveability problems very well. I will be sure to let you know what I find, so that hopefully it will help you as well.

 

mercedestech. that would be so awesome if you could let me know what it is when you find out. mercedes tech and you have a z? thats awesome. i bet you hate working on them mercedes! also now i noticed two or three days ago that the coil wire at the coil is arcing pretty bad. you can hear it when the car is running and can see it arc at night every so many times it fires. can this just be my coil and coil wire? i will be replacing them both as soon as i get the parts and time to do it. i really do believe this might be my problem. moisture and condensation after raining getting into the coil and causing it to not start sound right? please let me know man. email me at eac95z28@aol.com that would be great thanks man.