93 300E - Rough idle after 02 sensor, air mass sensor, OVP relay, wiring harness, etc

[dordal]

My 93 300e 2.8 still revs at idle, more than a year after I bought it. This site has been great, and I've tried everything suggested here, but to no avail. I'm hoping someone out there might have some more ideas.....

The Car: 93 300E 2.8. M104 engine. 170K miles
The Problem: Revs at idle, from about 600rpm up to 1200 or so. Its an intermittent problem. Doesn't matter if its hot or cold. Recently it also started idling rough.... the car now shakes at idle. Does this intermittently too.

What I've tried:
- New OVP Relay
- Checked Wiring Harness (was replaced at 99K and is OK)
- New Air Mass Sensor
- New O2 Sensor
- Checked for vacuum leaks, both visually and with starter fluid
- Checked coolant temp sensor; its OK

What I haven't tried:
- Replace engine speed sensor (I can't find this guy!)
- Replace throttle actuator (very expensive, and the car runs great on cruise control, which everyone says is one of the first symptoms of a bad actuator)

My questions for our resident experts:
1) Should I try the throttle actuator? Do these things go bad on the M104 engine?
2) Where is the engine speed sensor? How easy is it to replace?
3) Anything else I should be thinking about? A through search of the archives doesn't turn up anything, nor does a search of the tech manual.

David