OS X :: Hardware Cracks Concerning USB Adapter For A Headset?

I'm pretty new to Macs, I've tried googeling and searching this forum. I didn't find anything sounding like my problem, so I hope somebody can help me with this.

I use Teamspeak and Skype on a regular basis. I already knew that you require a USB adapter to use your normal headset, so I bought one by Creative (actually, a USB soundcard) when my new iMac arrived.

At first, everything seemed to work fine. I was talking to a few guys in Teamspeak when they told me I sound like Darth Vader all of a sudden. I doublechecked that by switching channels and doing the Teamspeak self voice test. It sounded really choppy, my friends were right.

I disconnected the USB soundcard, replugged it a few times. Eventually, it worked again. Well, it didn't prove to be for ever, since the problem came back - I can't say there's any special situation and I couldn't reproduce it. It was just random.

Ok, my second guess was that I would require different hardware, since Creative didn't provide any drivers for MacOS. I went back to the shop and bought a simple adapter by Speedlink, cheap stuff, I know. I just figured "less complicated hardware equals less problems". This Speedlink adapater just transforms the signal for USB.

MacOS found it straight away, and I started using my headset again. Well, cut a long story short, the same thing happened today, at random. My voice suddenly becomes chopppy and sounds something like Vader. I switch to internal Mic, use it for a few minutes, switch back (Sound Source is great), the mic works for a while, and then it all starts over again.

So, any of you guys have an idea what the problem is?

(before you ask, it's a Sennheiser headset, upper middle class, and has never failed me before)