I am sure Digs may or may not disagree with me
I
absolutely agree!
You don't have to pay ridiculous prices for over-priced luxury brand cables, but you do need quality cables with quality, properly assembled/attached connectors to minimize interference problems. Contact cleaner is not normally needed unless the contacts have been exposed to the elements, cigarette smoke, skin oils, etc., but it can't hurt (though some will "eat" some plastics). Often, just careful unplugging and plugging of the connectors a couple times is enough to scrape the contacts clean of dust to allow a proper mechanical connection which, in turn, provides a proper electrical connection.
That said, connections do wear out. The female RJ-45 (Ethernet) jacks have spring-tensioned tines that mate with the male contacts of the cable connectors. If the tension gets weak, the mechanical connection may get weak. And cable connections are easily damaged by physical abuse.
And shielding can also be essential if you live in an electrically "noisy" environment - depending on the wire. Audio and video cables need good shielding but network cable (CAT-5e and CAT-6 Ethernet cable for example) is UTP - unshielded twisted pair, and does not. You don't need shielding with UTP because the twists causes the wires to cross over each other at pre-determined intervals, which cancels out any interference that may be induced in the cable. But UTP cables
MUST be properly terminated with good connectors, properly attached.
Common sources of interference are AC cables so I always run my power cables up one side of my desk and data cables up the other. I separate my power and audio/video cables behind my home theater system in a similar fashion. Fluorescent lights, microwave ovens, wireless phones, and other wireless devices can create interference.
One of the best ways to eliminate, or at least minimize interference is to ensure your home and/or office wall outlets are properly wired and grounded (Earthed). I recommend all computer users - actually anybody who plugs anything into a wall - test their wall outlets with a
AC Outlet/Ground Fault Indicator Tester. They come in all shapes and sizes for the type outlet and voltages in your area.