iMac: Wi-Fi disconnects frequently

Information:

Product: iMac 27", Mid 2011

Processor: 2.7GHz Intel Core i5

Memory: 4 GB 1333 MHz DDR 3

Graphics card: AMD Radeon HD 6770M 512 MB

OSX : El Capitan 10.11.4

Hello there. Since Upgrading to OS X Mavericks, my iMac has ongoing connection issues with the Wi-Fi. At first, it would frequently disconnect, like every 5 minutes. The only thing that helped was the diagnosis tool in the network prefernces. But it was a real pain to do this every time my mac disconnected. After moving this gigantic machine to my router to upgrade the software to El Capitan via LAN (as it cannot be updatet without internet which is great if you don't have an internet connection due to the update), things even got worse. My Wi-Fi connection would be either extremely slow, or not even connected at all. I then tried to clean-install OS X El Capitan. For the first hours, everything worked fine but then the problem reappered. I did not even use a time machine backup so the computer should be completely free by any software that could have caused the issue. Until now, I have tried the following that I found on the web:

• OS X Clean Install
• Deleting files in "SystemConfiguration" folder, including preferences.plist, then rebooting
• Setting up new environments with Wi-Fi as the only available service
• Deleting all Networks under "
• Deleting all networks in preferred networks
• Configure IPv6 to Local-Link
• Using Google's DNS-servers instead of the default servers
• Automatic Configuration of the MTU
• Removing the "Thunderbolt Bridge" virtual interface, which would reappear all the time by its own

If any of you might help me, I am very frustrated that even the full clean install did not help to solve my problem. But as it worked before the Update on Mavericks, I do not think it is a hardware issue. I will attach what I usually do to solve the problem but I would much rather like a persistent solution...

Thank you very much and I look forward to get some helpful advice.

Kind regards

CleeTie

---Attachement---

Step 1: When my WiFi disconnects, I usually use Network Diagnostics to fix it. The first three points are always green, ISP is sometimes yellow, sometimes red and the last two points are always yellow.

Step 2: I choose WiFi and use my usual WiFi network.

Step 3: I Insert the password which is safed in my keychain.

Step 4: Every point except the first one goes red.

Step 5: Wi-Fi settings goes green. System asks me whether I use DHCP or PPPoE (how do I even find out?)

Step 6: However, before I can make a choise (between DHCP and PPPoE), the systems tells me that something has been changed. I press ok.

Final Step: Apperently, my internet connection appears to work. However, three minutes later I have to restart this whole process.