Wifi speed is always variable. distance/obstruction/interference is going to cause significant changes in speeds. Plus 2.4ghz is much much slower than 5ghz. when you say your wired speed is stable, that means the device is performing as it should Wired will be the most stable as it doesn’t have the interference, distance, obstruction that causes variability with wifi. 

So it isn’t a modem side issue. Or theoretically even a router issue based off the details you’ve given. 

If you think its an issue, more details are needed because your current description just describes how wired/wireless speeds function

Thank you for your reply.  Unfortunately my tv is the only device close to my S36 cable modem with wifi router. The rest of devices are connected by Wifi and other devices are connected through wifi.  If the internet speed is close to 300 MB all the time, I do not have any issues.  However, internet speed via wifi vary from 40~ 300MB. That is the issue.  Before Xfinity updates from DOCSIS 3.0 to DOCSIS 3.1,  S36 online status is DOCSIS 3.0 (Green LED status) and stable internet speed, but once they upgrade it to DOCSIS 3.1, then Wifi connection speed is unstable.  If I could set up S36 to support only DOCSIS 3.0, then I think WIFI speed is stable. 

Try disabling band steering and setting up different ssids for the 2.4ghz and 5ghz.. the G36 (you put S36 but there isn’t one) supports band steering. If a devices is on 2.4ghz, 40-70mbps is an expectation as 2.4ghz is a much slower network than 5ghz. 

Could be the devices are on the 2.4ghz when you’re getting the lower speeds and switch to 5ghz for the higher. Its not the router that switches them between bands but the actual devices own bandsteering/roaming protocols. 

I set up a different SSID for 2.4ghz and I will monitor the wifi connection speed for a couple of days.  Thank you so much for your valuable support and advices.  I really appreciate. it.