Hah!  Mysterious intermittent D channel failure was finally traced to a driver problem and FIXED!

I couldn't make it happen very often, even when driving the most likely scenario pretty much continuously for several weeks of a fortuitous lull in other testing needs for the SUT (of course, it needed the *big* SUT) and the Hammer.  And it took a while to convince my developer  that no, it wasn't a problem with the test equipment - quite understandably, the developer has more confidence in what's under his control, and less confidence in my test equipment, than I do.  But I finally collected enough clues to let him know where to look, and he found some a problem that would plausibly account for both the behavior and its low probablility of happening.

This gives me great satisfaction.