I've got the ATI Wonder Pro, and I ran into the same problem. All of the above replies are hitting the nail on the head, but, not really driving all the way into the wood (huh huh..he said wood..)..
First off, let's talk about where your signal is going NOW; crappy splitters do cause a great amount of resistance in the line, and, as a rule (of course) the more that you split the line from your original connection into the home, the less in signal strength you have delivered to each outlet (tv's, cable modems, etc..). (These signal strengths are measured in Db). Also, try to cap off any unused cable lines in your home, because the ends that are unused become small "transmitters", emitting that cable signal into the dead air, hence, causing more resistance in the whole circuit, and
could actually collect odd radio signals, injecting interference into the circuit, too (bad lines on your tv, whining noises as cars drive by, etc, etc..) You can buy RG-59 (that's the cable type for most cable lines in your home) caps from Radio Shack, relatively cheap to solve this.
Second, let's talk about what to do when you have thirteen thousand television sets in the home, that the cable guy didn't install, but you did. That's totally legal, as long as you have done it "inside" of your home, and not fooled with the splitter that they have installed on the outside of your home...You now need to get a signal (db) amplifier. The Radio Shack equipment SUCKS (big, green, hairy, nasty ones). They're cheaply made, and do not contain the better made circuitry that eliminate the boosting of other shit like those odd signals that I spoke of earlier in this book...Ebay (spam, I know) has a dealer that stocks some pretty good deals on proffesional grade line boosters/splitters and caps (amoung other thangs) I've had good luck with his guy's equip..From here, you can contact him and tell him your prob, and he can help you out fairly quickly and honestly w/o any hassle.
Last, as you go to purchase one of these amplifiers, keep in mind that the tv connections in your house only "receive" a cable signal, whereas your cable modem receives and transmits a signal to and fro the cable co- If you experience drops in your connections over the internet, then your signal(s) to and fro the cable modem need to be amplified, as well- In this instance you will need to purchase a bi-linear amp.
Hope that I could help, but really, check this do0d for your problem solving needs..
lookie here. (I have no affiliation with this cat, he's just helped me create some wonderful tv in my home!)
Hope that I could help.
-t