That's a brilliant idea, I almost forgot about this. When I installed networking gear in the 90s, common knowledge/best practice was to always fix duplex/link-s

That's a brilliant idea, I almost forgot about this. When I installed networking gear in the 90s, common knowledge/best practice was to always fix duplex/link-speed on fixed installed gear (routers/switches) because they frequently messed up autonegotiation. Try a managed switch where you can choose 10/100, Full/Half and see statistics. Also try to get a "netstat/ifstat" like display on the logic analyzer to see bad packets/collisions/...