Slower WiFi Speed after reinstall

Hi,

I reinstalled Mountain Lion 10.8.2 with the recovery menu for 3 weeks.

After I tested my WiFi speed over 5 GHz with WPA2 I found out, that the download speed is "very" slow.

So I tested it with Windows 7 x64 SP1 with Bootcamp.

I've a 100 mBit/s connection and I can get the full bandwidth with my PC over LAN.

My router is a Belkin N750 DB - and I don't think that this is the problem (see below why).

Before I reinstalled ML I reached 75-90 mBit/s over 5 GHz WiFi.

I tested it at speedtest.net with the same server (Norderstedt, Germany, Hosted by Willhelmtel - this is the nearest)

The results are following:
Mac:

Download = ca. 25 mBit/s

Upload = ca. 28 mBit/s

Win:

Download = ca. 85 mBit/s

Upload = ca. 28 mBit/s.

I tested it over one week, every day around 5 tests.

The problem seems to be at Mountain Lion.

I have a MacBook Pro 5,3 Late 2009 with 8GB RAM and a Samsung 830 256GB SSD.

Is that a known problem?
Is there any fix?

Please, excuse my english - it's only german school english 😀

Greetings from Hamburg

Jannomag