R6400/AC1750 sudden drop in wireless speed

Have had an R6400/AC1750 running my wired and wireless cable (Medicaom) connections since 10/2016 with no issues. Did a firmware upgrade a few months ago with no consistent/obvious problems, although a few wireless slow-downs in the subsequent period that I could not pin down. Did notice that there were suddenly issues with the MAC address being required, and had to mirror one from a PC (which it periodically loses). Yesterday I noticed really slow wireless. I pay for 100 Mb/s. If I connect directly to the cable modem, I routinely get about 130. Up until yesterday's measurement, I would get virtually the same wirelessly from the AC1750 anywhere in my house. Yesterday it was 7 (yes, SEVEN) Mb/s wirelessly.  I did all the rebootings, powered off for five minutes, etc. Still basically the same. If I wired a PC directly to the ports on the 1750, then good speeds, but wireless was next to nothing. I did a full factory reset on the 1750, and afterward got only 14 Mb/s. Seems there is something severely and suddenly wrong with the wireless transmission speeds. I've got about a month under warranty for hardware, and this is sure feeling like a hardware issue. Any thoughts or previous experiences/resolutions on this issue with the 1750 would be appreciated. I currently have my old Netgear WNDR3400 running and it's delivering about 80 Mb/s wirelessly, so it's definitely an issue with the 1750.