slow to non existant connection speed on brand new iMac.......

I'm hoping somebody can help me, I purchased a brand new iMac the other day, brought it home, registered and connected it up to my router, it was all very straightorward and I was using the machine exactly as I has expecetd to for the first 24 hours, the internet speed was good.

The following night the internet then significantly slowed and initially I had just put it down to bandwidth so I didn't think too much of it despite the fact that my PC was uneffected. Today I lost connection entirly and although it did return return once or twice for a short while, the sopeed was so slow that I couldn't even load a simple web page.

I spent ages on the phone to Apple support today who tried various things but ended by suggesting that I take the machine back as it may have a hardware issue. I have today exchanged the machine for another brand new iMac, took it out of the box and I have exactly the same issue. I can sometimes connect to the inteernet although when I do the speed is unbelivebly slow, at time it often says that I have no connection.

At all times the machine appears to be connected to my wireless router, a router which also connects to a 2 PC's, a PS3, an iPad, 2 iPhones and a WiiU all with no issues at all. We are never using any more than 2 of these devices at any one time, quite often it's just the one as there are only two of us in the house.

Does anybody have any ideas why/what is happening? Any help given greatly appreciated, thanks in advance.

James