Problems with microphone

[Ramfan]

I recently purchased a Droid Pro from the online store.  Incosistently the internal microphone will stop working and the person on the other end cannot hear me.  Everything seems to work fine when in speaker mode.  I have done factory restore and removed battery several times.  The microphone may work in the beginning of the call but then begins to fade.  My face does not accidently engage the mute.  Any suggestions?