SOLVED - Connection slows down 10-15 minutes after restart. Restart fixes but slows down again

I just wanted to share how I resolved my problem - as I see many reporting this issue. This might not be the same issue as yours and might not fix your problem, but worth a try. 

I recently signed up with XFINITY new 100 mbps plan from my earlier 75 mbps plan. Since the upgrade , I had connection drop issues. 

I have my own Netgear CM500 modem and an AC68U router. 

I got only speeds of 30mbps on speedtest. When I restart my modem the issue resolves and I get speeds of upto 100mpbs over Wifi. But within 10-15 minutes the connection speeds slow down to 30mbps. This behavior was pretty consistent every time.  I also verified that the behavior was the same even when I connected my laptop directly to the modem bypassing the router. So I ruled out the router to be the issue (I should have tried this before updating firmware on my router)  

Called XFinity Technical support, but everything was showing green at their end. And they couldn't help much as I had my own modem. 

As a last resort I tried to factory reset my modem, and that seems to have resolved the issue. As I am writing this I have 24 hours of consistent 100 mbps speed over WiFi so far. 

Do note that I DID not use the hard reset button on the back of the modem. But I used the web interface of the modem admin page. 

I will update here, if I start to see issues again.