Update: I checked the last 3 components I hadn't checked: the EHA (20.5 Ohm, OK), the altitude sensor (couldn't find it, my car may not have one), and finally, the throttle position switch. BINGO!!
The swicth activates for full throttle, but not for no-throttle. I jumpered the wires, started the car, and it ran OK. Now the problem is that the dealership says my part number doesn't exist, so we looked for the US-car equivalent (remember, mine's Euro). Apparently,they don't make the switch anymore without purchasing the entire *throttle body*. Does anyone know where I can get a switch?
BenzMac? I'll search this site in a minute, but I'm a bit pessimistic.
For what it's worth, my part number is 006 545 1824
Thanks for all your help... after 3 months of diagnostics, I've finally got a lead!!