Thought I had a failure of the microphone on my headset (http://www.davidclark.com/HeadsetPgs/h10-134.htm) at first on our flight yesterday.  Wiggling things around up at the headset itself made it cut in/out, so I thought perhaps it's just a wiring issue with the headset (and not an airplane issue).  Very odd since they're not that old and don't have that many flights on them.  Finally figured out that the microphone itself screws into the boom and if it's at all loose, it becomes inconsistent...  New item for the checklist...  Check those microphones before flight to make sure they're tight!  :-)