Since it's WiFi, even more reasons for fluctuations.
Did you try the old set it to 802.11g mode trick?
As the headline says my download isn't constant but my upload is.
My download speed differs between like 100Kbit to 25Mbit constantly while my upload speed is always 4.5Mbit, always, it never changes.
I am using windows 8.1 64bit and my wireless card is a Atheros ar9485wb-eg and I have tried different drivers without any luck. My other laptop is getting a fairly constantdownload speed of 22-25Mbit and they're not far from each other. The modem/router is just a few meters away.
Has anyone got an idea to what might cause this?