IMac slows down wifi/LAN cable connection

Question

Hello,
I came across a quiet unique and annoying problem, that I have not seen being discussed so far.
I spend the last 3 weeks trying to solve my internet/wifi connection following all kinds of posts here.
Problem: Wifi/Lan internet connection is slow (less than 0.8mbit/s)
Problem only appears when I login with an iMac via wifi OR lag cable
iMac, 2011 27`` Mac OS 10.7.5
Background:
I have a 20mbit/s internet connection with my provider. works fine on iPad, iPhone, appleTV HTC one mini, etc… BUT as soon, as I login with my iMac, the internet connection slows down to less than 1 mbit/s
I have tried following solutions:
- live box (router) was exchanged and is brand new
- security settings on router changes (wpa2 etc)
- all channels on router tested from 1-13
- keychain saved passwords for Wifi and router deleted, PRAM reset
- renewed lease of connected devices (no IP conflict)
- shut down wifi on iMac - connection with all devices great, as soon as i switch wifi on, all devices lose their high speed connection and go down to less than 1mbit/s
- connected iMac via LAN cable: same problem, all speed is lost
- iMac is not in the network - speed is great
- connected iMac via time capsule wifi, same problem. connected iMac via netgear extender, same problem
also, if I open network preferences and start the "assist me" app and run diagnostics, all dots are green except the ISP and server, bit most of the time they switch to green immediately, but connection is still slow.
Reception bar is on "3", so signal strength is no problem, and I´d like to emphasize that this is also happening on LAN cable. and again, if the iMac is disconnected from wifi AND / OR LAN, all other devices work fine.
Would love to hear your thoughts!
Thanks!

Answer 1

Although I have been on Mac OS for a while now, I have never had to troubleshoot faulty network in the mac itself.
OSX does apparently have a safe mode boot..
OS X: What is Safe Boot, Safe Mode?
Try that.. as that gives you the same thing as windows with all the extras left out, so you get a basic operating system only and you can see if the problem still exists.. if it does.. I would simply bite the bullet.. wipe the Mac.. reinstall the OS and then carefully reinstall your applications from original disks/downloads and move your operating files back .. but start a different user on the computer and copy back the min you need to get working again.
If the problem still happens even in safe mode boot then do a hardware test.. because it is possible something has actually failed at the hardware level.
Using Apple Hardware Test