I have my cable line coming from outside to my listenig room where I split the signal to my cable hdtv box and to my computer modem
Why did you split the signal when you could have had your cable company run you a dedicated line for your HSD ? Which is the way it is suppose to be. Was the HSD in another room and you took upon yourself to move it someplace else ?
More than likely, the splitter that you are using is not allowing forward and reverse to happen which you need to have with HSD.
I would call your cable company and have them run a dedicated line to where you would like it, and sure they might charge you for this, but it is only a one time fee , but will be worth it.