W2K3 downstream (Internet) performance

Hello,

I had a W2K server connected to a high-speed
internet connection, and I had about 40 Mbps
downstream speed in average.

Then, I clean-install a W2K3 and connect it
to the same connection as above, but now,
I only get about 8 Mbps downstream speed
in average. I have tried any possible tweak
regarding tcp/ip, but no luck.

I have found out, that with W2K3 SP1 (since
build 1218), downstream performance problem
seem to be fixed, but I seek for a solution
for current build (RTM).

Any help will be appreciated. Thank you.
--
Yunarso Anang