Eventually got things fixed. The CATV grounding was good, tied into the electrical grounding system where it enters the house.
The solution involved 3 separate fixes, each of which improved the hum somewhat, but all 3 together were needed to eliminate it completely.
First, the audio ground loop isolator I had ordered was installed in the audio line, and this eliminated the buzz from the audio.
Installing a similar device in the coaxial RF input to the cable box (which connects to the TV via 5 RCA cables for audio and component video) greatly reduced the severity of the hum bars, but didn't completely eliminate them. The device I used is here:
http://www.amazon.com/VSIS-EU-Cable-Ground-Loop-Isolator/dp/B0017I3K9MEliminating the last traces of the hum bar was accomplished by running a 10 AWG THHN grounding wire between the PC chassis and the chassis of the TV. Getting a connection to the TV chassis required removing the back cover of the TV, and attaching the wire to an internal metal shield.
Thanks to all for the input!