C7000 suddenly has slow internet connection speeds

I purchased the C7000 Nighthawk modem/router on 3/16/2016 as an upgrade/combination of a previous Motorola modem and N900 Netgear dual band router.  The Nighthawk performed flawlessly until last week, when the internet connection speeds (both wired and wireless) became noticeably slower. I have the Extreme 105 plan from Xfinity (105 MBS download, 20 MBS upload), but after running Speedtest.net, I only saw ~15-20 MBS download and ~7-8 MBS upload.  I checked with Xfinity and they said everything was normal on their end.  I have looked through a majority of the Netgear forums here and have done everything suggested:  power cycling the modem/router and network, changing wireless channels (several times), rebooting the modem (via Netgear Genie app and routerlogin.net), and performing factory resets (both Netgear Genie and routerlogin.net initiated and putting a paperclip in the reset hole on the modem).  During some of these steps, the performance improved somewhat, but it quickly degraded back to very slow. 

I wanted to see if there are any other suggestions out there - my Netgear hardware warranty on the Nighthawk expires in one month and would like to know if this is a sudden hardware problem, where I can replace the Nighthawk, or it is something else.  Again, this is a relatively recent connection problem; the Nighthawk performed flawlessly for the previous 11 months.  Thank you.