uneven performance Options

[df]

i've been noticing that my DNS accuracy varies quite a bit. on a good day, I can get approximately 90% accuracy. But then I may do something else, come back to dictation and the accuracy drop significantly.
For example, I may unplug the USB pod, plug it back in, and suddenly I am at 70% accuracy.
My system:
DNS 7.3, win XP running on a Pentium 4 dell with 512 MB RAM, GN netcome mic headset with VXI USB pod.

several questions:
1. what causes the accuracy fluctuations?
2. i've got an onboard sound card (SoundMax digital audio), and I have selected "mute" for all the settings under volume control for fear that it may interfere with my usb pod/microphone. is this the right thing to do?