Will the iperf3 test can be influenced by several factors it shouldn’t be fluctuating as much as you are showing.
Did you always had the same target server which was not doing other CPU heavy or network stuff?
Does ifconfig show any errors, drops or overuns?
As I wrote in my HowTo when both sides are wired with 1000Mbit you should expect a clean 940 Mbit.
So looking at your numbers there is definitely something wrong/strange.