[RP-PPPoE] [OFF?] auth issues? Mtu? what?

[Tiago]

Hello all,
I'm having a strange situation here...

1) Few customers/desktops after authentication (pppoe) get low
download rates way lower than the configured ones. Eg: 25mbps, most of
time the customer will get 3mbps, but it can get 25mbps for short
times.
2) If we take a router and connect those desktops to the router it
will work perfectly, 25Mbps all the time.
3) We measure rates with ftp/http downloads and with speedtest on our
local network.
4) We got one of those desktops and tried: linux auth (OK), raspppoe
(OK), but with the built-in pppoe (windows xp) and one (windows vista)
we can't get it OK. We already changed the network card.
5) We have 10 computers with the same windows xp version, and just one
get the same results as explained above....

I don't have any idea about what can I do more to find whats
happening. We're receiving a lot of complaints from few customers
having the same behaviour.

Any idea friends?

Thanks