Audio Recording: Voodoo?

[verduron]

G3 - Blue and White - 300 MHz - 876 MB / MAC OS 10.3.4
Recording audio cassettes to digital format.
Using Griffin Tech USB iMic.
The problem: after a while the sound becomes very scratchy and 
distorted.
I was told to take the Ethernet cable off. I did, I recorded a couple 
of 90 minutes cassettes without a problem.
But it started again. After rebooting the computer I thought it might 
remove the problem, but it did it again, after 50 minutes into the 
recording.
Using a separate peripheral hard drive for the recording.
There is nothing else on the drive. I did not de-fragment it, just 
reformat it before recording.
I am now trying to record, using a direct connection to the computer. 
Not using iMic.