iMacs internet connection fails every 5-10 mins

the mac runs smooth but after 5-10 mins the cinnections fails. It says its still connected on the top right where the wifi settings are. When i run a diagnostics check it works for a while, then the ISP turns red along with the Internet, and Server. Its not the modem because my macbook and iphone are working perfectly on the wifi. So why does my iMac keep doing this?