WLAN Internet connection issues, timeouts (WinXP works, Win7 not)

On my old netbook with Windows XP 32bit the WLAN works perfect. But I encounter connection issues on my new notebook with Windows 7 64bit. I'm connected to a router.

Symptoms:

• playing games works pretty fine, despite some heavy lags now and then
• messenger (MSN, ICQ, IRC) work fine and stay connected
• FTP fileupload with FileZilla works only in active mode, not passive mode,
  uploading files in passive mode result in endless timeouts,
  since active mode opens ports on my computer, is it a port-based issue?
• wireless internet on other machine with Windows XP works
• I experience the problem the most when browsing the internet in a webbrowser, if a connection issue happens then no websites will load anymore
  most of all I got problems connecting to DeviantART (graphic websites with lots of images) the website will load but rarely the thumbnails of the images, the progressbar just loads and loads
• EventViewer on Windows often gives messages about timeouts during DNS lookup
• when the internet connection in the webbrowser drops I can still ping google in command line, using the websitename takes ~20ms, using the IP takes ~40ms, I can also still chat in my messengers

Trials (nothing solved the problem):

• disabled IPv6
• running commands:
  netsh interface tcp set global rss=disabled
  netsh interface tcp set global autotuninglevel=disabled
  netsh int ip set global taskoffload=disabled
• set router encryption from WPA2 to WPA
• tried an older driver for my WLAN adapter
• tried different firewalls, also disabled firewall and antivirus completly
• installed all possible windows updates, also silverlight
• disabled the wireless adapter of the notebook and plugged in an external WLAN adapter dongle

I looked around for solutions of this problem and found alot of people talking about similar issues (change from WinXP to Win7, suddenly having connection issues on Win7 while WinXP works fine), but I did not found THE solution yet and I would wonder if there is any, cause then it would have been already included in a windows update I guess.

It really seems to be a Windows 7 problem, I really hope somebody can help me. This heavy problem can drive someone back to Windows XP and waiting for the next major release. Despite this problem Windows 7 works perfectly, I love it and would not like it to go back to Windows XP just to be able to browse the internet properly.

Thanks for any answers and links.

I use the free Avira Antivirus and free ZoneAlarm Firewall.
I completed the Immunization of Webbrowsers by using Spybot Search and Destroy.
Windows Firewall is turned off completely.

Scan of C-Drive using Avira Antivirus Scan : nothing found
Scan using Spybot Search and Destroy: nothing found

Internet Troubleshooting Guide for high-speed/broadband-cable connection:
IP adress valid,
ping <Default Gateway> successful,
ping Google successful

Starting Windows 7 in safe mode with networking, no firewall or antivirus, notebook is placed next to router:
same behaviour as with normal windows startup, internet connection is dropping/timing out from time to time,
FTP upload in passive mode not possible

Using an external WLAN USB dongle and disabling the internal WLAN adapter:
same, problem remains

First eventviewer message I got after booting Windows in normal mode after save mode was:
Timeout (3000ms) while trying to connect to "Atheros Bt&Wlan Coex Agent" service
(Co-existence Coordinator Service between 11a/b/g/n Wireless LAN and Bluetooth.)
But enabling the service did not changed anything, since it is something about the communiation between WLAN and Bluetooth it seems to be unimortant for my problem, but I wanted to mention it.

I can connect to the internet, but the connection is often dropping for several seconds/minutes which effects less my chat and messengers, but most of all the webbrowsing. The problem is not that I cant connect, the problem is that my connection keeps timing out. Best example again is the FTP fileupload. The webbrowsing often may work, but the FTP upload in passive mode keeps timing out. Maybe part of the problem is the size of the requests im sending to the net, since I can always ping websites. Even when my webbrowser is timing out on google, I can ping it successful as much as I want. And the network icon of windows never says "not connected", I'm always connected.

My Mom and Dad are using Windows 7 too in our homenetwork, but as far as I know they never experienced problems like me yet.

System File Checker: No integrity violations

WLAN Adapter: Atheros AR9002WB-1NG Wireless Network Adapter

Browser: I previously used the Firefox 10 Portable Edition and other portable Browsers. But since your last post I'm using the installed version of Firefox and it seems that fixed my issue. I guess the portable version of Firefox caused the timeouts and lags. Since I'm using the installed version it is running without problems. Maybe someone else who is using the Firefox Portable want to comment on this thread. I also want to mention I experienced similar lags with the portable Firefox from time to time on Windows XP.

For me this issue seems solved, thank you for your time DustSailor!