In order to save yourself tons of time, there is a very quick way to see if a problem is inside your house or from the cable company.
WARNING: Disconnecting the ground block can be
dangerous if your electrical system is not grounded properly! If you suspect
your home wiring to be old or in very bad shape, do NOT remove the ground block
or you could potentially be shocked!
Take your smallest TV out to your ground block, and hook it up there. If you are still experiencing a problem at the ground block, call the cable company and have them fix it. If your problem goes away, then you have an internal wiring problem and you need to continue troubleshooting from the junction (if you have one).
You may be asking yourself how this applies to cable modems. Your cable modem requires a much cleaner signal than your TV does. If you have a crappy picture on your TV, your cable modem is going to receive a crappy signal as well, which can cause retransmits, slower performance, dropped connections, etc.
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