If your board was made in the U.S. it's quite likely someone who performed the mundane tasks, e.g. cleaning the boards post-assembly, was intoxicated in one way or another. With signal impedances as high as they often are, a little surface contamination can cause all sorts of mysterious effects.

Has anyone "rung out" the board's interconnections? That can point out a number of things, too, some having to do with manufacturing quality.

I mention this not so much to impugn your PCB/assembly house as to remind you that it's easy to check for and fix this sort of problem.

RE