Wifi - is it a mac or isp issue?

I am using an iMac with yosemite. I have been having constant connectivity issues lately. I have noticed that my wifi is constantly switching from 'looking for networks' to 'on' . I thinking this is causing internet drop outs, slow response times when clicking on sites etc, but I can't be sure.

We have bad internet signal issues here, 3G on phones is only one dot, and the router is in a different room, but the portable mac which is in the same room as the router seems to have the same problems. Is it something in the mac, or does it should like a problem with the router or ISP?

It is so frustrating I just don't know what to do. Telstra says there is no drops out on the line, but I can't seem to run any downloads as they all seem to drop out within the download time, progressive intervals.

Any advice appreciated.

Veronica