slow upload speed

Can anyone help me out? For the past couple of days I have been experiencing very slow upload speeds. My upload speed sits at around 40mbps, but out of nowhere it will drop down to LESS THAN 1. I am currently under and UNLIMITED plan and don't see why this an issue. I have visited an Xfinity store and they mentioned that we had an old modem then proceeds to replace it with the same gen we had. After setting it up we STILL was had slow upload speeds. After returning back to Xfinity to explain to them we were still experiencing back speeds they then gave use the newest gateway, and guess what we still have terrible upload speeds. I myself love to game and because of our upload speed and cant even open a game. Our upload speed will drop to no joke 0.05 and wont return for maybe over 4 HOURS. PLEASE HELP ME!!!