The flaky one-channel-dropping problem resulted from an intermittent RCA terminated interconnect cable. It was NOT the cable from Dish to the 3-way selector. It was the cable from selector box to Sonos box.
It was a typical cheap "throwaway" cable included with some piece of equipment I must have purchased. I use such cables in non-critical places where the utmost fidelity need not be preserved.
I threw it in trash and replaced it with another throwaway cable for now.
My old standard for such applications was the cheapest variety of "shielded" Radio Shack intereconnect cables. Radio Shack raised the price of those cables about 3x in the past 10 years, so I quit buying them so often. Then they dropped the 3 foot stereo variety, then they added extra fat RCA plugs on the ends, which are clumsy and often difficult to insert in tight places. You can still buy the famous old style cable in mono singles (rather than stereo pairs). I just bought one pair of such 3' mono cables last week. I think I'll get lots more, maybe do some tests first. Belkin PureAV cables also have especially fat RCA plugs...so fat and difficult to insert they are virtually useless. I bought a bunch of those then regretted it.
Radio Shack now also pushes many different varieties of upgraded cables, but quite often Radio Shack branded upgraded cables often have plugs that are especially difficult to insert (I worry about damaging the jacks if it takes more than 5 pounds of force to insert), or cables that are especially stiff and difficult to use with lightweight equipment like a 5oz selector switchbox. I bought a bunch of Radio Shack upgraded cables before I discovered these problems.
The nicely shielded interconnect cables from Blue Jeans come with nice Taversoe connectors (spring loaded connectors hold tight but not difficult to insert) but are especially stiff and heavy (and not cheap).
Two major objective standards for interconnect cables are shielding and capacitance. I have meter for measuring capacitance, the old standard Radio Shack cables are OK in 6', but surprisingly the 3' variety had identical capacitance (around 220pF, IIRC, so that's <40pF/ft for the 6' cable, but <80pF/ft for the 3'). That's OK I think, though MANY cables have less capacitance (while some audiophile cables have far more).
Shielding is hard to measure, but I'd guess the cheapest Radio Shack cables have shielding that is mediocre at best. Single very open braid, IIRC, little different from twisted pair. But since I've never noticed noise relating to use of such cables, it's probably OK also. Not great, but OK unless you notice hum/noise from using cable. Virtually all hum comes either from power supplies in need of refurb, or ground loops, not the cables themselves. I know people who used twisted pair, or even spaced wires (!) for interconnects, and swear by them as "the best". A pair of spaced wires has virtually no shielding, in fact it's a loop antenna.
Another standard is dielectric quality, but you're not getting teflon (the best) without paying a lot, and second best polyethylene is generally only found in professional (e.g. Belden) cables or overpriced upgraded cables. Recently I've been using mainly Blue Jeans cables for most critical audio applications, and they use polyethylene (and have multiple shields and low capacitance).