New internet, both wifi and ethernet freeze entire system. Please help

Hello.

I recently changed internet providers from Orange to Movistar. The new connection is fibre optic 100mb and works amazingly well on all other devices and computers in the house.

I'm using an imac - OS X 10.6.8 - Intel core i5 2.8GHz - 8Gb Mem

The problem I'm having is a strange one and I can't find any solutions using my usually very helpful friend Mr Google.

The previous internet connection worked fine for both wifi and Ethernet but the new connection does not work at all. Even more than not working, it freezes my entire system when connected.

Here's what happens;

With airport turned off and Ethernet cable unplugged, the machine works superbly as normal.

As soon as I turn airport on or connect Ethernet cable I'm unable to click on anything without the spinning wheel appearing. It never goes away and requires a hard reset. Every aspect of the machine is unresponsive including keyboard short cuts. Of course, to make to the machine usable again I have to turn the router off, restart the machine and turn airport off before restarting the router.

I've asked a friend who works in Australia as apple support, he recommended I try a few thing, like creating a new user log in and creating a new network location. I have tried both and his next suggestion was taking the machine to the nearest apple shop for a service. The problem is I live in Barcelona and do not speak Spanish. I've been in to the shop without the machine and tried to communicate but it's difficult for both parties and I'm not sure they understood the problem. I'm reluctant to carry the machine all the way to the shop if its not absolutely necessary.

If anyone can help me I'd be very appreciative.

Thanks

Tom