During a huge file transfer between 2 ubuntu dapper (up-to-date) machines, i use to display the netspeed graph on both machine for various reasons. Knowing that both machines only do the file transfer (no other perturbating network task that could "pollute" the graph values), I think that the polling of the device is bad. It could be something else, but The graph is misleading : it goes often over 100Mbps, and the physical link is a 100Mbps.
One machine has a rtl8139 100Mbps card.
The machine which displays a "faulty" graph has an e1000, clamped to 100Mbps because the switch (Netgear FS116) is a 100Mbps one). Network cables are new, it tested with other cables if that could be a problem.
Both machines use last -686 kernel.
Note : doing the same file tranfer between my machine and another e1000 machine (a server without X) produce the same sort of graph ("bad" peaks).
I attach two images, one from the rtl8139 machine, which I qualify as "good" graph, and one from my laptop (e1000) with bad peaks.
I could guess that the average value of the bad graph is good, I mean averaging the peak and the bottoms we could obtain the real network trafic average value, because the bottom values are smaller than those on the rtl8139 machine.
good graph on the rtl8139 machine during a file transfer