could there be some kind of virus, ghost that would cause this ?
I am saying this because the same internet connection ( wifi) works very well with an other mac ( in wifi as well)
188.8.131.52 is a Google DNS server which will be one of the fastest anywhere in the world - DNS server speed can be checked with NameBench.
You should be aware that each time you change your DNS server your DNS cache gets flushed so don't be surprised if a future DNS server change appears to be better. I arrange for my DNS cache to be automatically flushed at boot. I boot once a day.
I am not a wireless user and was interested in my daughter's recent discovery about her wireless link to her cable modem in Washington:
"When I logged in tonight at 10.00pm I was getting slow speeds again. I looked up in 'about this mac' and saw many wireless networks, many using the same channel. I then changed my network preferences to only use 802.1n rather than g or b (largely because most other networks seem to be using g). This then gave me a new set of channels to chose from - I picked one that no-one else used and bingo... my speed is now consistently 50Mbps! At least I now know what to do to fix it. I was probably having the same problem in my old apartment block, but never associated the gradually increasing number of wireless networks with my slowing internet speed."
My UK telephone line restricts me to about a third of this speed - see: http://links.zero.eu.org/radsl/