I'm using a Motorola Surfboard SB6121 on Time Warner Cable.
SB6121 is a good modem, but is a older 4x4 D3 model. DOCSIS 3.0 (D3) takes your traffic and devides it over multiple frequencies. Each frequency is a different channel. Like lanes on a highway, the more you have, the more data you can move over it without being effected by traffic. Since the Sb6121 only has 4 channels, if 1 channel gets congested or noisey, 1/4 your bandwidth drops away. New modems have like the Arris/Moto SB6183 and Netgear CM500 have 16 channels, the CM600 has 24 and the brand new SB6190 has 32! If your speeds aren't consistent through the day, and is worst during high use times, then upgrading the modem may help.
See here for a full list of TW modems.
Not to say the modem is the problem. Have you done any tracert or pingpaths to the game servers you play on? Many times the game will have support info showing you how.
What games do you play?