5Ghz Band Not Stable

Looking for some last ditch help to get this router to work. I've spent more than 12 hours tinkering with settings and continue to get the same results. I have Comcast with GigaBit internet. Earlier this year, I was able to get stable speeds 200+ mb/s on the 5 Ghz band. Recently the router reset itself and I cannot get stable speeds.

On the 2.4 Ghz band, there are no issues. I'm able to get a stable 100 mb/s on this band. I've tried differnt channels, different modes, I've tried QoS, and I've reverted to the V1.0.1.114 firmware from V1.01.118. Still no luck. I know it is something to do with the router because, I've hooked up an ethernet cable and I'm able to get sub gigabit speeds. 

At this point, this is just a glorified 2.4 Ghz router. Has else experienced similar issues on the 5 Ghz band? I'm really close to giving up and searching for another router. Let me know if I can provide other pieces of information that might help discoving the root cause.

Thanks!

Ron