Have to restart Mac from Wi-fi to cable internet

Hi!

I have recently (April 2013) purchased a MacBook Pro. I have, however, recently encountered a stupid problem with the internet!

Whenever I take out the internet-cable and switch on the wi-fi (if I for example wish to use the computer in another room!!), there is no problem. However, when I want to go back to using cable (and I therefore switch off the wi-fi and connect the cable to it) I have to restart the Mac in order for it to work. This was not a problem a couple of weeks ago but I have experienced it now.
Anybody know anything about this annoying thing..?

What are you doing that's different from what it was a couple of weeks ago? Have you changed anything or installed any programs that might effect networking? When changing over from wireless (WiFi) to ethernet, the changeover should be automatic without having to reboot the machine.

Well, since it is a new computer, I have installed quite a few things. However, nothing new since this problem started to occur. Though I have to say that today it did not seem to have this problem, as the transition happened automatically!!! Strange!!!