Bob,
Take out the crossover and re-solder every connection. What you're describing is common to a cold solder joint.
Allow me to swerve with a little rant here. There is No Such Thing as a "cold solder joint". They are all "cold". At least when the soldering iron isn't touching them.
What you are trying to describe is a fractured solder joint where there is a complete crack going all the way around the lead. The solder rapidly oxidizes and you are left with what amounts to an oxidized switch contact that makes intermittent contact under vibration. What most people call a "cold" solder joint is often a "dewetted" solder joint. Where the solder did not wet out to the surfaces it was trying to join. Sometimes this may happen from insufficient heat, thus the misnomer "cold" but more often it is a result of un or marginally solderable surfaces due to oxidation or some other contamination. There are variations of this. From non-wetted where the solder never wetted, to dewetted where the solder recedes from a marginally wetted surface, to poor wetting where there is some but there are also areas of dewetting which can create stress risers leading to fractures down the road.
My other favorite is looking at reports from the production test department attributing functional failures to "insufficient solder". As long as the solder wets both surfaces there is an electrical connection. As long as it doesn't get blown open by high current like a fuse, a minimal solder joint works perfectly. It may not have as much physical strength, but that is often another misconception. Once you have a smoothly wetted joint connecting both features, more solder does not make it better. When you get to the point that you can't see the shape of the original surfaces you were joining, you've put too much solder in there. Excess solder makes it harder to determine if the wetting is good and can create stress risers where convex solder joints meet wires, leads or terminals creating less reliability. The only exception to this are certain SMT components where the joint is on the underside and a convex external fillet distributes the stress from a fracture in the thin underside solder, prolonging the service life of the joint. Which will crack eventually. I know this because I did the original research back in the '80s and drew the pictures that were added to the military weapons grade soldering specification and subsequently copied into the commercial industry soldering standards.
We now return you to your regular busted crossover...
