If you are splitting your cable more than once or twice you probably need an amplifier in there, although an amplifier usually will make things like on-demand, ppv, etc not work properly (so in my house we split one to tv, then one to amp/splitter to another 3 tvs and cablemodem).
Also, plug the cable directly to your TV and watch it for a bit (no modem, splits), see if it still has a problem, if so call a tech in to check the line.