Idle Problem Solved
Just a followup post to close this thread out: I replaced the throttle actuator today, and it FINALLY solved the problem. Figures it would be the expensive part. The idle is now pretty stable -- it sometimes hunts just a bit when switching from D or R into P or N, but stabilizes pretty quickly. I assume this is normal.
Special thanks to Arthur Dalton and Paul S (suginami) for thier help on this one.
If you've got the same problem (doesn't everybody?!) on the M104, try the throttle actuator. I'm going to disect my old unit when I have some time, and see if I can figure out exactly what went bad. If anybody's interested, I'll post details about what I find.