Internal Mic is working a bit then stop working.

Hello!

My iPhone 3G internal mic is working great when getting out of sleep mode, but about 10 to 20 seconds later, the signal become fuzzy then quit working. The only way to get some signal is to yell straight into the phone location (getting a distorted signal). Then if I wait 1 to 2 mn, I can reproduce the problem and so on... Very frustrating. Any idea ?

I've already try to change the dock connector getting exactly the same results.