Unstable Wireless on Netgear R7800

Hi, I have recently been experiencing very unreliable wireless connection and it makes it very difficult to have a voice call. The speed profile over WiFi (either 5 or .24GHz) look like below. I stopped all services I was running (samba, sqm, adblock, etc) and it did not fix the issue. I am running the latest 17.01.2 and I do not think I was having this issue with the previous release, but I cannot be completely sure. I tested over a wired connection, and there is no issue there. There were no other users using WiFi during the test.

Any suggestion about what I can try?

There is definitely nothing in the log and as far as wireless config goes I have only added this "option disassoc_low_ack '0'"

I ran this test below 10 feet away from the router and used 5GHz band.