poor running mystery

[93_Clubman]

Have you checked for fault codes - if not:
http://www.miata.net/garage/faultcodes.html
http://www.miata.net/garage/Engine_Diagnostics.html

[StanTheMan]

all these years I was under the impression AU delivered NA6/8's didn't have that feature unless you go under the dash & install the light.....

[manga_blue]

You really have stated all the typical symptoms of a blown head gasket. In your case there's probaby combustion gas blowing into the coolant and then coolant being sucked back into the combustion chamber on every stroke.

Virtually any small garage has CO detection kits these days, where they check for the presence of CO (combustion gas) in the coolant. Sometimes they'll find it in the top of the radiator but more often it shows up in the overflow tank. Either way it means blown head gasket.

The test itself takes about 1 minute and most workshops do it for free. I wouldn't bother chasing difficult electrical and sensor problems until I'd eliminated the most obvious solution.

[speed]

^^^ this.
Was the head reconditioned, shaved or at least confirmed flat before the gasket was replaced?

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[93_Clubman]

all these years I was under the impression AU delivered NA6/8's didn't have that feature unless you go under the dash & install the light.....

Stan, from memory the LED & resistor is required for the ADM NA6, but the engine check light on the ADM NA8 is enabled.

[Code4]

Was the head re torqued after the new gasket was installed ? Sometimes done after a few hundred kms in older engines.