It's kind of like you move toward the mic to start talking and then quickly start to move back. If you're using a headset, it's possible the program is automatically adjusting the volume down after you start talking. Assuming you're usng Skype, I would check the volume setting there, since it's possible you have auto-volume adjustment on (which is good) but have the volume slider below the input meter too far to the left (which is bad). As a result, the volume is good to start, but then Skype adjusts it to be much softer. Just a theory.
By the way, how much dynamic range compression are you using?