Infuriating internet disconnect problems with iMac (Mavericks)

Hi,

This problem has plagued my iMac for about 3 years now. Here's the back story...

Back when I had OSX Lion installed, when connecting wirelessly it used to disconnect from the internet constantly, sometimes every 5 minutes and sometimes I'd get a couple of hours out of it. I finally cured that problem by changing my router security to WPA2.

OSX Mavericks also worked fine using the existing settings until we got our internet upgraded to fibre optic 2 months ago. Now the thing disconnects every 20-30 minutes. I was so frustrated that I bought a pair of "Powerlines" and decided to connect to the router using an ethernet cable in the hope that it would stop dropping the connection. It hasn't made the slightest difference, it still drops the connection every 20-30 minutes.

All other devices in the house have stable connections, including my 2009 Macbook which is also running Mavericks. I think this is a serious problem suffered by huge amounts of iMac owners judging by the number of threads I've seen over the last 3 years.

The latest problems coincided with having the new fibre optic internet installed. As soon as we had that installed I am getting the constant disconnects again. But... there can't be anything wrong with our internet as all other devices work perfectly. It has got to be something wrong on this iMac. Would buying a 3rd party wifi dongle cure the problem do you think? I am willing to try anything to get this problem cured.

Can anybody help?