I have a 1985 380SE Mercedes that has been a dream for me up until now. I have had a problem for about 1 month where my idle will sometimes be at 400-600 RPMs when I put the car in Neutral, and at other times it revs up to 2000-2300 RPMs when I put it in Neutral. I have found no pattern for this and have checked all lines, tubes, and conntections.
And to top that, it has just failed Virginia Emissions with the following readings:

PREVIOUS CURRENT
HC ppm 15mph 68 241 Failed
HC ppm 25mph 108 253 Failed
CO% 15mph 0.38 3.47 Failed
CO% 25mph 0.61 3.54 Failed
NOppm 15mph 28 303 Pass
NOppm 25mph 31 332 Pass
RPM 15mph 612 1398 Valid
RPM 25mph 434 1210 Valid
Dilution 15mph 14.8 16.5 Valid
Dilution 25mph 15.0 16.5 Valid

I have not been able to get anyone able to tell me what the problem is. I believe the O2 Sensor should be replaced, but I need to keep repairs to only those necessary (money tight due to medical bills) to pass emissions here in Virginia.

Does anyone have an idea so I can deal with this and not make unnecessary repairs at this time?