I bought the game some weeks ago and had similar issues right away from the first installation.
Extremely choppy and unstable downspeed, going down to well under 1MBit/s fluctuating and so on.
No other program or download has this problem.
For me, the solution was to set a specific setting in windows as per this thread:
There are many suggested solutions there and i tried most of them, however the only one that worked for me was to set this in a cmd-shell:
netsh int tcp set global autotuninglevel=experimental
After setting this, i restarted the pc and tried again and it worked.
Maybe this works for you. If it doesn’t you can always set it back to normal (netsh int tcp set global autotuninglevel=normal)
Here’s my post from that time in the same thread:
Since then the sim downloads constantly at my full connection speed.