I reported last week about a veeery strange network problem here (See post 1 & post 2).
Friday morning, before everbody arrived, I replaced one of the 3cSOHO100-TX Hurricane network cards with a D-Link one we had lying around. That interface then worked without any problems. I digged some more but could not find a solution. Though I could confirm that the problem was not around with Linux kernel 2.4.x but only with 2.6.x. So I had two options: downgrading the kernel or replacing the cards.
The most pragmatic solution was to replace the network cards and I did so in the evening after school was over. Some tests showed that the problem was indeed solved. Today, Monday, I looked at the MRTG statistics and saw that the problem was indeed solved.