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Car in question is a 1993 Mazda Miata with 1.6 liter engine.

Right after I start the engine the RPM drops below idle and the engine shakes a little bit but quickly recovers. After I give it a very small amount of throttle, barely a touch, the RPM also drops. Sometimes, but not always, after holding the throttle at a certain RPM and letting off the gas the RPM's would drop.

As well, after the engine has been running for a long period, at least a half an hour, when the car is stopped the engine would "shake" every 10 seconds or so and I can feel it in my seat. It is not as violent as it sounds.

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    Any black / white smoke out the exhaust? CEL? Overheating? I had identical behaviors on my car (not a Mazda), and replacing the air filter, the MAP sensor, and a cracked intake pipe cleared it up. The 93 Miata has an AFM, dunno if the symptoms would be the same, you could try cleaning it. Another thing to look for is vacuum leaks on the intake, look for holes in the intake tube, cracks in the mentioned plastic pipe between the AFM and throttle plate, etc. Also the other stuff in that article.
    – Jason C
    Sep 21, 2016 at 4:43

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Sounds like trouble with the idle speed control valve. On the '93 Miata, it looks like this: enter image description here

This little guy, when working properly, will let enough air into the engine to keep the engine alive when you are not using the throttle. Sometimes they get dirty, sometimes the connector gets loose, sometimes they breakdown and need to be repaced.

Here's what you do:

  1. Find out where it lives on your engine.
  2. Remove the connector and test the electrical resistance across the pins on the valve itself. You should get 11-13 Ohms.
  3. If the reading is out of this range, the valve is electrically defective. Replace the valve and see if your problem disappears.
  4. If the reading is within the range, the valve is functioning correctly electrically, it may be mechanically sticking open or close. Remove the valve and clean the inside of it. There's a spray cleaner you can buy to clean electrical sensors.
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