If you decide to buy a CATV isolator use the Viewsonics one, it is designed to work with 2-way services. (As a very temporary test you could connect two 75-ohm to 300-ohm matching transformers back to back; I don't know that this will work properly and allow your cable modem to sync, plus it will result in signal leakage; however as a temporary test it may be OK. The matching transformers should act as an isolator. But they're not shielded at all so....).
That in conjunction with an isolation transformer should solve the hum problem you are having.
