Intermittent Internet Connection

Hi guys, new member here. I am writing in hopes that someone can help me solve my problem of having intermittently slow internet connection. I am getting extremely frustrated staying on the phone with the router people and Comcast to no avail. Most days my internet is fine, I show a strong signal and fast speeds opening pages. Other days, like the last two...I want to rip the remaining hairs out of my head. My speeds are earth shatteringly slow or I can't connect at all.

Please tell me the first place to start, thanks in advance!