Think I put this in the wrong place to begin with…
2005 Launch Edition 1.8, with idle drop problem. Every time I came to a standstill the revs dropped off to almost nothing before the ECU caught it and picked it up. Been getting worse for a while so I decided to investigate the Throttle body.
Throttle body easy enough to get off, undo the two big air hose connections and move that out the way, undo the four bolts holding the throttle body to the manifold and the two coolant lines (might get a bit of leakage but not much).  Unclip the electric connection and it’s off.
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If it looks like this when held up to the light it’s not good. - there should be a clean path for air on idle past the butterfly valve.
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I cleaned mine with STP carb cleaner - there was no coating or anything on the valve so could get it nice and clean…
Compare the difference after cleaned - how much more air’s getting past the butterfly at its idle setting.Â
Two coolant pipes back on, four bolts, a connector and two big hoseclips and it’s running like a dream. Rock solid idle with no droop.
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Everyone with idle issues should read this thread
Top man Stew cleaned throttle body late last night drive to work and no issues idle is great no up and down issues fixed
Interesting stuff…
My 2006 2.0 NC has been playing up in a very similar fashion.
It went back to Mr M - they changed the plugs…still the same at idle.
When I pushed Mazda to have it back and fix it they looked through all the old technical bulletins and found a bulletin that detailed precisely the problem I have ( you had ).
The Mazda fix is to remove and clean the throttle body.
I googled the TSB’s and found the one in question, worth having a look at as the suggest the body is removed leaving the coolant pipes attached and cleaning in a specific manner. Link to TSB below.
Mine is back in on Friday - collecting it on Saturday morning - hopefully all fixed and sorted 
jr
http://www.miata.net/garage/tsb/01-007-09-2069.pdf
For info, with the throttle body not powered up, the butterfly is in it’s fail safe position not it’s idle position. Failsafe gives about 2500rpm, the butterfly is closed slightly by the PCM to achieve idle speed.
Yes exactly how I cleaned it no need to remove coolant pipes plenty of room and again I drove home tonight and its still working all fixed and it dosent take long to do this job