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What does it mean when I'm idling at the lights and my revs go right down to zero, but the engine doesn't cut out? It was like that for half the trip this morning on the way into work before acting normally. Same thing happened on the way home too.

Would appreciate any advice.

How did I manage to get this in the wrong section?!?

Mods, please move to Engines.

I had an erratic tach needle in my NA8 and turned out to be a loose ground screw on the throttle body.. Tightened it up and problem went away. Can't remember exactly where it was, will try to find a link.

Just had a quick snoop at your garage thread and its a NA6 right? So still a B6 engine? For the NA6 the rev signal is sent from the ignitor (its on the right side of the engine, next to the brake master). Unfortunately i'm not sure if this is a common issue at all. It will be quick and easy to swap it out with another from someone to test if thats the fault.

Also try to clean all your ground points in the engine bay, especially the one under the brake booster, inlet manifold and under the washer fluid reservoir. There's also one on the alternator.

I sometimes experience the same problem, which only occurred strangely after changing the spark plugs.

I've cleaned some of my grounding points and it is less frequent.

Also double check that all your ignition leads are properly inserted to the coil and are not arcing on the rocker cover. While you're checking that do a once over on the plugs connected to the coil pack.

+1 on all the above. Guran provided some links here from when he had the same problem:
viewtopic.php?f=55&t=43726&p=594423

If that doesn't sort it, then check the insides of the coilpack posts where the plug leads fit in as NA6 coils can have an issue with pitting & burning. Poor earthing can also affect the ECU, which can show up internally as a burnt earth post. Might even be the dreaded idle dip, so worth resetting base idle after disconnecting the battery to clear ECU error codes if any:
http://www.miata.net/garage/KnowYourCar/S7_Idle.html

Thanks everyone! I'll work my way through all of the above.