Ok, after months of just dealing with the problem using both a Mac and a PC, I went ahead and put my SBG6580 (Time Warner) modem into bridge mode. Now I'm routerless and wireless is gone, but at least I don't get the constant errors I got with the router enabled in the modem. Here's how to put it in bridge mode...
1. Unplug the cable from the modem.
2. Press the reset button and hold for 30 seconds to reset the modem settings.
3. Log into the modem via your browser at 192.168.0.1 using lowercase admin/motorola as username/password.
4. Go into Basic - Setup and copy the LAN mac address.
5. While you're in Basic, go to DHCP and set DHCP Server to No and click the Apply button.
6. Go into Advanced - Options and paste the mac address in the passthrough field and click Add Mac Address. This allows the modem to bypass the router and talk directly.
7. Check the RG Passthrough on the same page and click Apply. Your modem will reset. This disables all settings pertaining to the router.
8. Once your modem resets and you can get a login prompt at 192.168.0.1, go ahead and plug your cable back in so the modem can download its settings. It'll take a couple minutes for it to configure itself and come back online.
Be sure to invest in a wireless router to protect your system since it's now open. Putting your modem in bridge mode also disables wireless, so you won't be able to use it; another reason to get a wireless router.
This morning, before bridging my modem, I was getting -50 and -8008 errors repeatedly on my Mac downloading multiple TV episodes. After bridging the modem to bypass the router, every file downloaded without one error.
If you ever want to reverse the process, it's easy. Just do step 2 (reset the modem) and then configure your Wireless password and port passthroughs, you should be off to the races with the router again. One note about port passthroughs with the regular router mode on the modem---I found that if you specify a range it must start with 1, not 0. Even though there is an option for BOTH (TCP and UDP) I found that this option didn't work, I had to add an entry for TCP and UDP separately.