The SA could not replicate the problem(!), so I retrieved my car on Saturday (and the problem was indeed gone), but less than 8 hours later, it was back. The SA had asked me to bring it in if I could replicate it, so I did this morning, and took the technician out to show him.
Bad news was that I stumped him, as he indicated he had never seen this type of problem before, and was going to contact the manufacturer directly to see if it has happened anywhere else. He did state, though, that it could be a software problem, but at the least now they know I was not making it up. He noted that there was almost a complete drop off in acceleration in 2nd gear, and that it was sluggish in 1st (something I had not picked up on).
I hope it is merely software, but the earliest they could look at it would be Thursday, so more update later. Better yet, as I was driving away, and idling in traffic, the engine overheat warning light went off. It may have been due to the way we were pushing the engine during the test, but I am hopeful we're not opening a Pandora's box on this one.