Well,
The dealer's senior tech poked, prodded and tested this morning. It appears I have a small vacuum leak around cylinder 6, either due to a cracked (plastic) intake manifold or a bad injector seal. Since the precise location can't be determined back there without disassembly, we are going to replace both the seals and the manifold next week when they can get hold of one. Total cost is far less than a throttle actuator.
The tech showed me that, in fact, the actuator is compensating properly, but under high vacuum conditions, apparently air is getting in along with the fluid he used to try to pinpoint the leak. From older vehicles I have dealt with that had vacuum leaks, I never would have assumed that from my symptoms. I guess it is because of the much more complex nature of the modern injection system controls.