I tried to do something simple that has turned out to have problems.

I have two Kingston hubs, older ones, with 10baseT ports and BNC
connectors for thin-net on the back. I ran about 100 feet of cable
between the two hubs (from upstairs to downstairs).

The upstairs hub is connected to the cable modem via one of the rj-45
ports (straight, not crossover), PC connected to another port. Then
using the BNC on the back connects to another hub's BNC port.

It works, sporadically. If I leave just the cable modem, nearby hub
connected, it runs perfectly (my setup for years). Connect the two
hubs via the cable and it works for a while, then it all dies.
Disconnecting the two hubs doesn't fix until I reboot the cable modem.

I bought new rg58 cable for this, and used T-connectors and 50-ohm
terminators that I've had for years and used to use successfully way
back when.

The hubs are the Kingston 8 port WG hub and Kingston 8 port stackable
hub. The latter is not the one for which Kingston still has
documentation on their site, unfortunately. Mine has stacking ports
on the right of front, and master/slave and terminal unit switches on
front. The model for which docs exist is different. I've tried many
combo's with no effect. I set it for standalone mode (switch 1 on)
and terminal unit (switch 5 on).

Is it bad cable? Bad terminators? Or likely a hub problem?

Please send suggestions ...