really slow internet when I connect my iMAC

internet drops on all equipment in the house if I connect my new Apple iMac.

Ive tried plugging directly into the LAN socket and tried just using the wifi but each and every time my internet drops between 0.4mb to under 0.75mb in the house.

if I disconnect my iMac my internet is back up to 1.75mb

I'm in the country so its the fastest internet I can recieve

I have problems even watching Facebook videos on the iPad or the Now tv if the mac is connected as it affects everything.

the mac is only a month old and my pc or other equipment never had problems before I changed over

Any suggestions?

Thanks Ronnie