You need to isolate the issue and figure out whether it's a TWC issue or some settings on your router(s), or something running on your computer(s) in the background that is causing your download speed issues.
Here's how I would approach the issue:
First things first, take a look at the Event Log in your SB6183 (you can get into the router by typing 192.168.1.100 in your browser) and see what's been going on, i.e., any unusual entries that are labeled "critical". Then look at your Status Page in the router and see whether you have a large number of correctables and uncorrectables (which can be indicative of line issues, headend hub issues, line attenuation, etc), and also be sure to look at your download channel signal levels. Power should be between -15 and 15dBmv; any SNR levels above 50 are real cause for concern and you should get TWC out on a truck roll to service your line if you see that sort of thing. Same with download channels where power levels are higher than around the upper 40's; anything over 50 and your modem is working way too hard to overcome noise and signal loss issues. Could be in your house, could be too many splitters between the cable drop and your modem, could be the wrong kind of splitters, corroded lines, bad cabling, i.e., could be almost anyting. But your modem signal levels will tell you a lot.
Run some speed tests with just the computer connected to the router and see what you get. You'll have to watch the sequence of how you connect things: First, turn off the router and turn off your computer. Boot the router up first then your computer. If you can't get connected, don't worry, just reboot your computer again and things should work fine. If you can get 300/20 when just connected to the modem, then you'll know where to begin looking, i.e., your router and its settings.
On the router side, you need to make things really, really simple: Disconnect everything except one computer that you know is clean and virus free, and connect it to a LAN port, and then connect the WAN port to the modem (assuming you've ruled out the WAN side or the modem as the source of the issue). Then you need to strip back the router to a firmware that you know will provide you with solid connectivity; you're not looking for fancy features here, so I'd go with a version of stock firmware that you know is relatively stable. Make sure to do a factory reset so you get default settings so you get basic connectivity. Remember, the goal here is to get a working version that gives you optimal download speed results first. Then you can try some speed tests, tracerts and ping tests to see whether you're having issues getting through to specific sites; try various DNS servers (Open DNS, Google, Level 3, etc.).
Tools like Pathping (run from a commond line) or WinMTR, which you can download at Sourceforge will give you a lot of info on where potential packet loss and other speed-related issues might be occurring.
Assuming you can get one computer to function within the 300/20 tier you're supposed to be receiving, then you can start adding other devices to the LAN; don't forget to test things with wireless clients, one at a time also.
I hope you get the picture here that there are so many different things that could be causing your issue that there's no way anyone can give you specific advice on which one to look at first. Your task here is to isolate the issue so you know whether it's one of your client devices causing an issue, the router or its settings, or your modem, and perhaps on the WAN side in which case it's TWC's issue to resolve.
Believe me, everyone who subscribes to TWC has been through this to one extent or another and there's just no other way to resolve the issue other than to isolate the various component parts of your network and test, test, and test some more.
