I'd check the firmware version on the cable modem, and check your web browser for un-necessary toolbars and perhaps clear temp internet files.
It should be no problem accessing your cable modems web admin page from behind your own router, the fact that they're on different IP ranges is not a problem, since it's a different network (192.168.100.xxx)..your router will direct that traffic out the WAN port since it's a different network than the inside (192.168.1.xxx).
The fact that you were able to access it at least once shows it's not a network design issue, else it would not work the first time period. So don't spend time chasing down routing tables or firewall rules on your router.
Sounds like the cable modem is about ready to jump off the cliff. Replace it with a good Motorola Surfboard!