100Mbit capped network speed

Hi there,
A few months ago i was close to losing my ultra 4 with os 4 due to corruption in my os partition.
As debian etch is really old, and seeing that os6 works well with those older units, i replaced my ultra 4 with a more powerful unit (acer ac100 bought really cheap, with xeon e3 quad core cpu).
since then, i've upgraded my ultra 4 to os 6.10.2, with the goal of having an up to date backup nas.
as i've recently bought newer hdds, i finally got enough space to do a full backup. But this is where things get wrong :
- with rsync i get 10-11MB transfer when pushing from my server
- same with ftp, both with upload or download
- same with iperf (95Mb)
- thinking it was my nas that got slow, i tried locally with dd, but it saturates my hdd (140+ MBytes for single disk) in reading or writing.
- with os4, with ftp, my speed was around 800-900mb read and 400-500mb write

As the speed is too close to fast ethernet speed, i suspected an error of the negociation, but my prosafe switch reports gigabit ethernet too.
Suspecting a faulty nic, i used the second one, with the same results...
Several rebooting (including the 6.10.3 update) didn't solve anything either.
Network isn't faulty, i changed the cables, even connecting without the switch, without result.

Any idea ? (before considering the dilemma between going back to os4 or accepting having only 100mb capped speed)