I recently replaced a leased cable modem with an SB6121 that i purchased connected with my extreme and my speed got even better than with the old modem. No issues with consistent speed either. When i switch the modem, with the help from my cable provider, i changed nothing on my extreme. Maybe you got a bad modem?
Cable modem speeds will decrease over time as the speeds are reduced to accommodate historical signal/line conditions.
If the speed falls below that which you're paying for, contact TWC as it's their problem, not your modem's.
They can easily run a test on your line which will reveal issues on the line.