Hi @Anonymous User
The NowTV Stick hasn't got the most powerful wifi antenna inbuilt into the Stick body and using your WiFi extender seems to compound it even worse where i am guessing your wifi is not the best around your home if you have invested in a wifi extender.
I can only recommend the basic stuff, such has try a different wifi channel, see if 2.4ghz works better than 5ghz, try a HDMI extender flylead to move the Stick further away from the casing of the back of the television which should improve the wifi signal.
Out of curiosity if you have another TV in the house (closer to your main Router) does the BT Sports App run more stable with the NowTV Stick or there is no difference ?
I do own a NowTV Stick, but i tend to use my NowTV Boxes wired over Ethernet which gives me a pretty decent streaming experience when using both NowTV and third party Apps on the boxes.
Don't know if there is anything in this trouble shooting guide makes any difference and does other Apps such has YouTube etc exhibits the same symptoms has the BT Sports App on the Stick ?
Should it be only the BT Sports App that suffers the buffering on the Stick, then maybe try deleting the App and try a fresh install from the Stick Apps Store.
https://help.nowtv.com/article/troubleshooting-now-tv-box-stick
