I had this problem off and on for literally about three years. Comcast kept trying to pin it on my modem and router (I own mine because fuck paying Comcast a rental fee). I replaced my modem AND router twice within that time period with top-of-the-line newer models and the problem persisted. They sent dudes out in trucks to check inside my apartment and the outside lines. But for the outside line appointments--this is SO COMCAST--they couldn't actually guarantee if/when anyone would show up, let alone confirm after the fact that anyone really came. Unfuckingbelievable.

I ended up having four or five techs come to the apartment and just hit gold with one of them who seemed to actually know what he was doing, AND I had a ton of luck on r/comcast believe it or not. The tech told me my condo building at the time (30 years old) had some kind of wiring that gives out because blah blah reason and bottom line, unless the condo association was going to replace all the utility wiring in the buildings, I was probably SOL. I posted in r/comcast (where Comcast techs lurk and respond!) and one of them actually took my case on from some private office from the Comcast headquarters in Philly and confirmed what the good tech told me, basically.

When I moved here and the problem STILL PERSISTED, I just immediately started calling techs to the apartment again. Here, the problem was both that my cable jacks were split weird AND that Comcast was basically lying about how up-to-speed the lines were in the Downtown/waterfront area. One tech was like "yeah, so the lines need a ton of work and there are plans to upgrade them very soon, but right now it's bullshit and even my service sucks."

Funny, I had a bunch of outages in the summer (July, August) and Comcast said it was due to line work. My service has been perfect since then.

Call them, be aggressive as fuck, ask to have service credit for the hours you couldn't be online if you want (I always do--especially because I work from home a lot!!), and if it persists, definitely get a tech out (it's free unless they install something or fix something that isn't on them) and maybe post to r/comcast if you need super-duper help.