Have CM400 modem with upload slowdowns that happen at regular intervals

I have made contact with Comcast several times through their customer service chat. I got through to one rep who seemed that she genuinely wanted to help. She issued me a repair ticket number that we could take this entire issue under and said that she would look into my symptoms. I waited a day and wrote them to ask how it was going and in that chat (with a different person)they said they would call me the next day .The next two days came and went with no call and I chatted with another person earlier today who tho I gave them the ticket no. seemed to have no new information and suggested, like all the other reps did, that I get a new modem. So to satisfy them, I did. I went out and got a newer and better modem. plugged it in earlier tonight so as to make sure that the new modem was active at the time the upload speed is dragging so that I could make a contrast with the old modem turned on the apps that was using to upload, did a speed test and guess what?

 Problem persists.

 Soooo I have another repair tech coming over tomorrow afternoon. He's prolly going to push the comcast modem. at least as a test and I will probably accept it. I genuinely don't know what else to do. They keep saying their net is fine and they even implied in todays call to hook up the newer modem that they are reaching the limits of what they are doing.  They also don't do night calls with techs. So i can't demonstrate the flaw in progress. Oh well I guess I will try to explain it to the repair guy when he gets here tomorrow. Maybe he has an idea.