XR500 slowly loosing wifi speeds

So been having some odd issues with my XR500 since I got the unit a few months ago.  The main issue seems to have really shown up ever since I upgraded to Xfinity 1Gb service yesterday.  They sent me a XB7 gateway that is now in bridge mode and if you connect via hardline to the router you get the proper speeds up near 1Gb, but if you connect via WiFi you only get around mid 650Mb range and that eventually falls all the way down to 200Mb and then goes back up.  All within the span of a few minutes, but the wired works fine and stays pretty constant.  I have tried 3 different machines that have at least WiFi 5 speeds on them and they all do the same thing.  Plug them in directly and poof everything is fine again.

Now I did see a few posts here and there about the XB7 gateway having crap firmware on it and that most don't have any issues with the older XB6 gateways so I have one showing up tomorrow from Xfinity to test it out, but it just seems odd to me that the gateway/modem would be the issue when its only the WiFi on the XR500 router that seems impacted so far.  Reboots/factory resets help for a short time but do not resolve the overall issue of the slow down.  Its almost like there is a memory leak, but there is no memory leak that I can actually see in the DumaOS.  I am running the latest V2.3.2.56 as I am not in the beta for v3.0.

Any ideas or thoughts about what could be going on please let me know.

Thanks!