Internet slows down after short period of use

I have a Gigabyte brix gb-bxbt-2807 and have recently started to use Ubuntu 18.04 rather than Windows 10. Everything seemed to work well for a while but I noticed that online music seemed to be dropping out. I discovered that my download speed had dropped to .2 meg After a reboot this went back up 11 meg. This keeps happening but is only affecting this PC not other equipment. I am using WiFi and have’t tried a wired connection. Can anyone suggest a solution?