Windows 8.1 dropping/losing Wi-Fi/WLAN connection, limited internet access, frequently re-connecting to router/access-point

Title in german:
Windows 8.1 verliert ständig WLAN-Verbindung zum Router / Eingeschränkte Verbindung

A while ago my wife's Dell Vostro 3500 running Windows 8.1 started to frequently drop the Wi-Fi connection to the router AVM Fritz!Box 7390 (sometimes once a minute), requesting a new IP-Adress by DHCP, but still showing limited internet access.

Things I tried, which did NOT fix the problem:
- Updating to latest drivers
- Switching to an AVM USB Fritz!WLAN Stick N v2
- Assignng a static IP to the network adapter

How-I-fixed-it:
I was able to fix the problem by changing settings for the WiFi-Adapter in the Power Options and disabling IPv6.

First:
Windows Key + R, type control panel,
go to Power Options. Click Change Plan Settings,
click Change Advanced Power Settings, then Wireless Adapter Settings,
then Power Saving Mode. Change the Plugged in option to “Maximum Performance”.
If this is a laptop, you might have On Battery option and set it to “Maximum Performance”. Restart computer.
I found thsi hint here:
http://atechjourney.com/fix-wi-fi-keeps-disconnecting-in-windows-8-and-8-1.html/

Second:
In the properties of the Wi-Fi network connection, DISABLE Internet Protocol Version 6 (TCP/IPv6), as described here:
http://www.thewindowsclub.com/enable-disable-ipv6-solve-internet-connectivity-problems

Wi-Fi problems with Windows 8.1 are described in many threads, here are links to the ones i found:
http://answers.microsoft.com/en-us/windows/forum/windows8_1-networking/windows-81-wifi-dropping-andor-limited/87531d97-c148-4387-acda-5a31b38c069e
http://answers.microsoft.com/en-us/windows/forum/windows8_1-networking/windows-81-dropping-wireless-connection/88ef2187-38f6-436b-bc25-e92cf6fe05af
http://www.eightforums.com/network-sharing/33754-after-8-1-update-wi-fi-dropping-connection.html

LAN/Ethernet speed/throughput decreases / becomes very low over time wíth Fritz!Box 7390 and Linksys WRT54G in the same network because of conflicting routers

In my LAN, I have a AVM Fritz!Box 7390 as DSL router and a Linksys WRT54G acting as a WiFi/WLAN repeater and LAN switch. Everything was fine for years with this configuration on DSL 16.000 (Annex B).
After German Telekom upgraded us to VoIP in the Fritz!Box (Annex J), problems started. Turning all devices on, full LAN and Internet Speed was present at all computer in the Network. After some time, the spped in the local LAN decreased (transfer rate from PC to PC as well as from PC to Internet) to just a few KB per second.(I tested internet speed with Ookla speedtest http://www.speedtest.net/). Internet Speed for WiFi devices connected to Fritz!Box router directly was OK.
I found out, unplugging the LAN cable from the Fritz!Box router and plugging it in again brings back full transfer rates in the LAN between all PCs and to the Internet.

How-I-fixed-it:
In thesSetup-menu of the Linksys under "Advanced Routing" there's an option called "Operating Mode". It was set to "Gateway". I set it to "Router" as suggested by the help text. No more LAN problems so far.

 

 

In German / Auf Deutsch:

LAN-Datenrate bricht ein / wird sehr langsam in einem Netzwerk mit Fritz!Box 7390 (als Internet-Router und WLAN-AP) und Linksys WRT54G (als interner Switch und WLAN-Bridge)

 

Nach der DSL-Umstellung von Annex B (16 MBit) auf Annex J bzw. VDSL 2 ist in meinem LAN die Netto-Übertragungsrate im Laufe einiger Stunden von anfänglich 10 MB/s auf wenige KB/s gesunken, sowohl zwischen PC's als auch in das Internet. Geräte, die über WLAN mit der Frtz!Box verbunden waren hatten weiterhin die volle Datenrate ins Internet.

Aus- und Einstecken den LAN-Kabels an der Fritz!Box behob das Problem kurzzeitig, jedoch trat es nach einigen Stunden erneut auf.

 

Ursache/Problemlösung:

Im Setup-Menü des WRT54G gibt es unter "Erweitertes Routing" eine Option zum Einstellen, ob über den Router eine Internetverbindung hergestellt ist (Gateway) oder ein anderer Router im Netz das übernimmt (Router). Bei mir war die Option auf "Gateway" eingestellt. Umgestellt auf "Router" und das Problem ist gelöst. Seither keine Einbrüche der Übertragungsrate mehr.