Why does my microphone fade out after 2-3 seconds whilst using Skype and eluminate?

My headset microphone becomes muffled, fades and then dead after 2-3 seconds. Using the Skype call tesingt service reveals excellent sound for just 2 seconds then it goes. I have checked and reinstalled the Realtec Audio driver but it made no difference. Of course my troubleshooters tell me everything is working normally. Any suggestions?