Hi Skyflyer,
I joined pikey-air on the 738 this year and treated myself to a HMEC-25KA. I had the same problem that you describe from the get-go, and I called the UK service centre after a week. The (very helpful) staff said that this was a known problem and required a new microphone to be fitted. As the headset was brand new this was obviously a warranty fix, so I duly sent it off.
Annoyingly, it took 6 weeks to get it back, but I have not had any problems since it came home.
Details on the Sennheiser website.
Wally.