For cable modem users, make sure your line comes right from the pole or is a single connection from the junction box and is not split or shared. For best results, you can request that your Internet line and your TV lines are separated from each other. This gives the best results.
However, if this is not possible, then make sure the line you're using come direct from the junction box and is not an output on a splitter. If you get a line split and you connect your cable modem to it, you're assuring the modem will target down to the slowest connection possible because split lines add noise and lower the db. Also, if you must split your line between your cable modem and TV connection, the best splitter is a two output splitter. Anymore outputs than that will give you a lower db rating and degrade your connection speed frequently cutting you off from the CJ server.
IMO, the best splitter out there for CATV is the Monster 5 - 1000mHz splitter:
