In an earlier thread discussing chain wear here I had mentioned getting grit in a brand new chain which I considered unrecoverable. Some months ago I resurrected the zombie chain, given it a thorough cleaning and pronouncing it healed.
Six weeks ago I switched cassettes (11-36) and never could get it completely dialed in. I was considering trying my spare RD, lubing the cable, etc. when in a moment for tardy inspiration I got out my low tech chain checker: the 1% gauge dropped in easily.
Now this chain cannot possibly have more than 1500 miles on it, I be surprised at more than 1000 (I'm about to get book keeping religion). Maybe a simple green soak would have actually resurrected the chain. Maybe wife and I produce massive amounts of power when climbing, maybe tandems can just be hard on chains.
Lesson learned - when in doubt check the chain. It takes 47.5 nanoseconds to do so and I spent hours with the tandem on the stand trying to get the shifting dead on. This is a basic truth when troubleshooting anything; always, always hit the low hanging fruit first.