After the technican replaced 50 cents worth of coax1, my cable modem is feeling much better. The connection had been deterioriating through 2003, but finally got really bad around New Year, and has been more off than on since then.
The technician told me that they can’t diagnose reverse signal (meaning from the modem to the Optus node) problems on weekends, because on weekends there is nobody at the exchange. Moral: don’t book a technician to come out on a Sunday and expect him to fix anything.
1 Actually, it was just a 30cm cable, but if you add the cost of the connectors, it probably cost a whole 50cents.