Any chance you can replicate the test where it is bad, and get on the vero at the same time (new putty/ssh connection open) a packet capture (dump)? Start the tcpdump capture prior making the iperf test.
sudo apt install tcpdump
sudo tcpdump -i eth0 -s100 -w /tmp/output.pcap
And hit Control-C when iperf is done. Let me know where I can download it. If it’s not too large, I can send you my E-Mail.