Wi-Fi keeps going down on MBP 2015
Usually when the WiFi connection is still active but no data is flowing indicates Chanel crowding (to many devices/users on same channel).

If the WiFi is disconnecting that could be number of other reasons. But since you are talking about the speed going down...

When the MCS index is zero (0) then the channel crowding is a issue. The MCS needs to be at least 3 or more.

To see yours hold the "alt" key and click on the WiFi icon. (this is what I have)



Easiest test (fix) is to change the Chanel on the router.

You could also do a WiFi channel analysis to find out and then choose a less crowded Channel.

To change the Wifi channel on Pirelli DRG A125G router: