FWIW This is a handy link for decoding what those flashing lights on the modems mean:
http://broadband.modemhelp.net/cable.../U10C018.shtml
As far as the utilisation thing goes they are not the same issue, having the connection be 'stuttery' compared with a total loss of connection are different issues and downstream congestion, which is causing the issues in the other thread, cannot cause loss of connectivity. Only extreme upstream congestion may cause total loss of connection and there's nowhere on the VM network which is congested upstream to that extent (upstream speeds too naff to stress that bandwidth).
It would be useful to get a couple of things though.
Is there anything in your modem's status logs? Any Timeouts or similar? Can you check the log next time it happens and copy/paste it before you reboot please.
Other than that the MAC address conflict is unlikely but possible.