My imac keeps disconnecting from wifi every 5-10 minutes.

It is not a problem with my wifi as I have an ipad, iphone and laptop all connected fine. Turning the wifi on my imac off and on again fixes it temporarily. But after another 5-10 minutes, the same thing happens again, very frustrating! I also can't seem to watch videos in anything above 360p anymore either... If anyone could help that would be fantastic!

Thanks