Home internet throttling evenings!

Well had good speeds all day for a little over a year and now when evening rolls around lucky to get 8mbps.  Yet my phone with same signal strength level runs at 120mps+ so evenings end up having to watch TV at lowest resolution or use up my hotspot data on my phone.  Local fiber just offered 300mbps @ $19.95/month so at this point a no brainer on what to do.  Just a suggestion but if they are going to over sell the market to the pint speeds drop below 10mbps the service should be free!