R6100-Ethernet Connected Speeds Way Down with WiFi on...Up with WiFi Off !?

R6100-Latest Firmware-

Greetings....! New to the forum. Long but detailed read....and thank you for your responses in advance. I have had this R6100 for almost 2 years and all of a sudden......

I started browsing last night at 10pm. My desktop machine is all Ethernet. Connection is a cable modem, not DSL. The speeds were tanked.... 1.46 down and .12 up. Supposed to be 6.0 down and .50 up. No other devices sucking down bandwidth via either WiFi or Cat5e.  I unplugged the power on the SMC cable modem and my R6100 WiFi router and shutdown the computer. Waited 5 minutes. Powered up the SMC modem waited for it to initialize completely and did the same for the router and computer in that order. Tested speed again. It's tanked. 1.36 down and .22 up.

I took the R6100 router out of the loop completely and CAT5'd direct from the SMC cable modem to my desktop computer (it's a Gigabit PCI adapter). The SMC "link" modem light goes blue indicating I went from a 10/100 LAN connection on the R6100 to a Gigabit LAN connection on the desktop machine. (It's green 10/100 when my R6100 router is in the loop) Tested speed again. It's up! 6.2 down and .54 up.
Hmmm...
Repeated all of the above, alternating the sequences 2 more times and got the same results. Put it all back to the way it has been and this time used the WiFi "off" button only on the R6100 router. Tested speed again. It's up! 6.0 down and .50 up. Turned "on" the WiFi button. Tested speed. It's tanked. Lather, rinse, repeat 2 more times. Same thing....WiFi switch off= speed is up. WiFi switch on= speed is tanked. Went to bed. It's 1:30 AM.

Next morning and all day so far you ask?  WiFi switch is on= speed is up. WTH....?!  Beats the heck out of me, judge. The main IT guru at my ISP (small town provider) says he has only seen this one time before. He had to do it all to another customers WiFi router...reset, reconfigure, everything. I don't know what brand or model. That time the customer had to replace the router.

It has done this late in the day (10PM) for 3 days now and I am out of ideas of what to look for. Most of the time it all works with the WiFi switch on. When the speeds tank I just turn off the WiFi switch and the speeds come right back.

So, what are your thoughts?

Thanks for your time and help.