Odd distortion when recording audio

Hi all.
I have a problem when recording audio with ardour (and also with other programs like Audacious). Whenever the levels get a bit higher the audio distorts (at least that is when I notice it). I’m 100% sure that it isn’t the audio card or anything else (like the mic I’m using) that’s causing the distortion.
To be sure on this I cross-checked with Windows and there everything sounds perfect.
What boggles me is that I’m not getting any X-Runs when the distortion occurs and the CPU is also only marginally taxed.
Because of this I haven’t the slightest clue what could be causing this. I’m not even sure whether I always had this and only noticed it lately when recording vocals or if it was something that came after a Kernel update or similar…
Anyway. My PC is and AMD Opteron running on a NForce4 Mainboard. I don’t have a dedicated Audio recording harddrive (might this be it?). The audio card I’m using is a STaudio/Hoontech DSP24 which has an Envy chip. I’m using the Ubuntu Studio packages on an Ubuntu installation.
I know this obviously isn’t an Ardour-specific problem, but I didn’t know exactly where to turn.
Hope someone can help me avoid an reinstallation of Ubuntu Studio, which at this point would be my last option.
Thanks in advance,

what kind of distortion ?
are you talking about clipping ? how does the waveform look like ? what do the meters show ? in what bit are you recording, 16bit ? or are you using 32bit ? what is your soundcard able to output ? 24bit ? or 16bit only ? there are many things to consider here. You need to monitor what you are recording and check the levels (input, PCM, output).

I heard this before…

Try using Periods/Buffer = 3