Home internet speeds extremely inconsistent and connection drops intermittently

just a rant here and some info, have had home internet for about 2 weeks. Started at steady 100 mbps speeds up to about 120, was thrilled. Today stuck at 10 mbps and under all morning. If this continues I will dump them. I rebooted the router 5 times no luck. My phone was getting much higher reading 30mbps-100 mbps so congestion and priority are an issue. Customer service is horrible and a giant run around game. They cannot fix simply billing and account issues and just straight up lie to get you off the phone and end the call. Hate xfinity but considering them at this point as the cost is higher but in theory up to 400 mbps so might be my only option left. Ok sorry for rant, in Ann Arbor Mich. here. Clearly they have oversold and rolled out too aggressively and now congestion is a big problem. Greed.