Netgear WNA 3100 signal strength and speed drops after 10 seconds

Hi everyone,

I recently bought a Netgear WNA 3100, I used for years a WNA 1100 without problems, since I started to notice some disconnection, so I decided to buy a new 300N adapter. And there started my nightmare..
My router is a TP-Link TD-W8951ND, my wireless network is secured with a WPA/TKIP encryption. I use Windws 7 Ultimate 64.
I tried to install drivers and software from the cd, then i downloaded newer softwares and drivers from the Netgear's download section, but the result is always the same.
I can't use any version of the Genie software, everytime i double-click the application icon nothing happens, just a little hourglass next to the mouse and nothing else. Any software version ends with the same result, I just can't run it. So I decided to keep testing all the driver versions I found on the web, and I used the Windows 7 network's functions and properties to manage my Wi-Fi.
Windows can reach my Wi-Fi network without problems, i put my wireless key and the adapter connects to the router. But after 10 seconds, the signal strength drops from 5 green bars to 2, and the connection speed drops from 54.0 mbps to 11.0 mbps, where it stays. I specify, I'm not talking about a speedtest result, but I'm reading the infos in the connection status window, that you can open by right-clicking on the wireless network SSID and choosing "status". This means that my internet is very slow, slower than my old WNA 1100. Infact, if I compare the two adapters, i can reach from the same position 2 green bars and 54.0mbps with the old WNA 1100, and 2 green bars and 11.0mbps with the new WNA 3100. Even if I put my adapters next to the router, the new WNA 3100 always loses strength and speed, either at short distance from the router.
Speedtest is terrible, I can reach just 1.3mpbs with a ping over 150ms. If i try my old WNA 1100 i can reach 4.5mbps and a 80ms ping. My DSL connection is a 8Mbps Telecom (Italy), if I try a speedtest with a LAN cable I can easily reach 7.2mbps and 45ms ping.

Every driver version keeps me drop, I deleted, reinstalled, rebooted, registry clean, but I still can't navigate properly, and I can't use the Genie software on this machine. Could you please help me?

If you need some more specification just ask me, I will really appreciate your help. Thanks in advance!