My car has exhibited the weird idle oscillation (between 1000-1500 RPMs) several times in the past month. The common link between all of the incidents is that it happens the day after it has rained. It has only happened about once a week, and right after I start the car in the morning. The RPMs will go up to about 1400-1500, then the engine seems to shut all fuel flow off, until the RPMs drop down to 1000-1100. Then the engine jumps to life until ~1500 RPMs, dies, then kicks back in at ~1000. I noticed that when it happens, a hissing sound (like a blowoff valve or something) is coming from the back of the engine (towards the passenger compartment). The whole process repeats continously until I restart the car. Once I turn off and restart the car, the car runs normally.
I have a couple other issues that have to do with a surging idle. They are: low idle, surging idle while switching from P to R/D and when the engine is under load (a/c on), hesitation, rough idle, and a mostly non-operational cruise control.
I've read some of the older posts on this topic, and am worried that this could easily lead to a mechanic just recommending change the MAF sensor and/or the Throttle acutator without checking other, simpler things first. I wonder if the hissing noise from the rear of the engine could indicate a vacuum leak, or something else related. I just had the 150k service 2k miles ago, with new spark plugs and fluids. I was also having a hard start problem about a month ago, but that has sinced worked its way out on its own. Oh, and the wiring harness was replaced a year ago.
I appreciate any advice, as I plan to go to the mechanic in the next week or so.