It may not necessarily be the EZL unit. In fact, the people I spoke to told me that these units are very robust. I am not sure which resistance you checked, but I was suggesting testing the resistance of the idle valve with the engine off and the connector to the valve disconnected. If I recalled correctly is should be around 5 ohms. You should check the current through the idle valve, that the inputs to the control unit are correct, that the wiring between the inputs and the control unit are good, and that the control unit itself is good (unit is located inside the car). I am going to do all these tests on mine when it stops rainning!
The story on my car is that it was purchased by one of the teachers at my son's school, serviced by a Mercedes mechanic during that time before she sold it to me. She had not idea that the radiator was partially plugged up (not as obvious since water was flowing through it without any obvious restrictions) and it also had the vacuum advance to the EZL unit disconnected. It passed the state smog test (this was illegal since only a deviation of few degrees on the timing is allowed -the car was off by at least 7 degrees!).
So this is the story on the "professionals" who worked on my car before I owned it....This is not to say that there aren't many honest and knowledgeble mechanics out there - not just pats changer guys!