CAX30---streaming issue

CAX30 modem router, 

I am having streaming issues on one TV.  It goes into a buffering mode at times...to the point where it becomes unwatchable.

I have taken the following steps

1.  Went to a faster internet service

2.  Upgraded to the CAX30 modem/router

3.  Replaced the old TV which used a Roku device with a new smart TV.

4.  Even though Staples told me it wasn't necessary, I added an extender at my wife's insistence.  (This is the TV she's more likely to watch than I am...what choice did I have?)

Last night, over about a half hour period, I checked the download speed several dozen times.  The Mps varied from 247 to 350.  I was about 15 feet from the TV with a clear path.  It was dinner and I was able to check it repeatedly.  While it might have been a hair lower at the TV, the speed still would have been high enough for streaming.  Towards the end of that half hour, the TV started buffering again.  Oddly enough, during the time the TV was buffering, the download speeds were all close to 350.

If I call Xfinity, I'm sure they'll suggest that I switch to their (overpriced) modem and router.

I do not want to want to tear through the walls to connect directly to the modem.

There are TV's in two bedrooms.  One is almost never used.  My wife watches the other when she goes to bed.  It is a little further from the modem and has another wall in the way.  She has never reported a problem with it, but she usually falls asleep quickly.  I don't recall it buffering when I come to bed shortly after and turn it off.

Any suggestions?