For clarity, the number you're referring to is the physical link speed and not the actual download speed between you and the internet.
With that said, if your wifi link speed is changing a lot it means there is a lot of interference or competition for frequency bands in your home.
The source of this problem could be incorrect configuration of your wifi access point, non-ideal antenna positioning on your wifi access point, neighbors wifi, microwave ovens, physical distance to your wifi Access Point, obstructions between your Formuler and the wifi Access Point and other non-controllable environmental factors.
Your best course of action is to try and reduce the distance between your Formuler and the wifi AP. If that isn't possible try to reposition the antennas on your wifi AP and try to reposition the Formuler for better wifi signal reception.
If the above doesn't help, you or someone knowledgeable about wifi will need to do a wifi site survey in your home and reconfigure the wifi AP's settings to best fit your environment.
Good luck.