Internet Issues

My 3 year old iMac is doing strange things.
It keeps dropping the internet connection but it seems like it is usually for well known websites. Google, Facebook, Yahoo etc. It will tell me that the website does not exist, even if I was on it 10 minutes earlier. It will then eventually stop being able to view all websites. Once I restart, it is fine for about a day and then starts up again. It does this with all the major browsers. Safari, Chrome and Firefox.

Also, when I view Youtube videos, it loads fast. (I have really fast internet speeds.) But about 30 - 60 seconds in, it restarts. The second time it will play all the way through but it is super annoying.

My Imac is hardwired into the modem and I am 90% sure it is not my service providers problem because our iphones, Macbooks and iPads all work fine on the wifi account. It seems to only be my iMac.

 

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Any ideas on this?
This is an example of what I always get:

Oops! Google Chrome could not find facebook.com
Try reloading: facebook.*com
Additional suggestions:
Access a cached copy of facebook.*com
Search on Google: