I've added an update to the original thread on this issue. That thread is here:
<http://groups.google.com/group/rec.autos.tech/browse_frm/thread/fc8a368c77aeb0e8
/8e0f70ba76f77529?lnk=st&q=#8e0f70ba76f77529>

For the record, I'm repeating the final update here, in a new thread:

Since the MIL came back on again, the dealership took the car back for
more troubleshooting. The tech ended up driving the car for a few days
as though it were his own, which turned out to be an excellent idea.
This enabled him to see the problem in action.

The culprit? The ignition module. Apparently this puppy was
intermittently failing to operate the coil, leading to misfires and the
related DTC's. The malfunctions were sometimes severe, sometimes not,
explaining the erratic symptoms.

The ignition module was replaced last week, and the problems and DTC's
have entirely disappeared.

Thanks for everyones' help here.

I'm wondering why a faulty ignition module would not itself have set an
error code. Don't GM's engine computers monitor igniter output?