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Duplicate ACK and TCP Retransmission

I'm doing some scp transfers between an AIX and a Linux in two separated LAN (I have a VPN between them).

Sometimes the connection ends well, so I get 50MB/s. In many other cases, instead, the connections starts fast and then drops until it ends with a low throughput, around 4-5 MB/s.

These are the test I made (file dimension: 1GB)

Scp transfer

As you can see, there's some cases where my connection is very low.

I made some tcpdump AIX-side and I found lots of DupACK and TCP Retransmissions. This is my dump seen from the AIX: Dump AIX to Linux

Could you help me understand what is wrong?

I made also a dump from the firewall. Let me know if this may help

Thank you :)