When it hangs for me, I can only get it working again by disconnecting the device and restarting the Amiga and then reconnect the device after it has booted up and then restarting AmiTCP. If I just restart Poseidon, it will crash immediately after using the Ethernet connection again. It may be due to AmiTCP not shutting down properly when running 'stopnet' though since it complains when I run 'startnet' that something already exists.
I will try disconnecting the cable and also see if Miami works any differently. With the 3c589 PCMCIA device I could never get Miami to work at all, but will try again on this one.