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hi steve,
i wouldn't have a clue on your problem, but the mx5 factory in ringwood might be a good start!

My thoughts:

battery??

fuel??

Sounds similar to mine...thought mine seems to rev out fine when cold...

viewtopic.php?f=29&t=37077

I'm thinking Idle Speed Control (ISC) or Throttle Position Sensor (TPS) problems. Both are on the throttle body at the head of the intake manifold at front right of the engine.

Try http://www.google.com.au/search?hl=en&q=ISC++site%3Amiata.net&btnG=Search&meta= and http://www.google.com.au/search?hl=en&q=TPS++site%3Amiata.net&btnG=Search&meta= and spend the next month reading.

I have this problem with my fresh engine.
Seems to have no power when cold but will rev and run smooth..

Once warm it is fine

Lol, can't help you either, but have the exact same problem on my gf's NA6. The only thing I can suggest is wait till the revs drop to 1000rpm or less, and it will be warm enough and won't stall.

problem with idle/stalling like this could also be a vacuum leak. The engine Idles fine when coldstarting because of the auto idle bump or whatever it is that gets the engine running at higher revs to warm the cat up fast but after the engine reaches ordinary running temperatures this is no longer in effect and the engine stalls owing to the vacuum leak but will rev freely for the most part when you're on the gas. Spray a bit of start ya bastard/something else that will increase the revs slightly (NOS?? haha) to pinpoint where abouts in the engine bay it is, then try individual hoses till you narrow it down. Otherwise take it into a mechanic, they can be a bitch to chase down. Could be as simple as a hose has fallen out/cracked or there could be a nigh on impossible to see hole somewhere

*edit sorry I don't think I explained that very well but my money would be on idle leak. Don't go messing around with the idle control etc unless you know exactly what you're doing

Given it's an na6, i'd suggest one of the two coolant sensors at the rear of the head, but it could also be an air leak that's intermittently presents when cold. Last one of these, which happened to be an na6, it turned out to be an air leak in the breather pipe from the cam cover - see 2nd last post of p3 of following or p1 ref coolant sensor:
viewtopic.php?f=29&t=35192

I seem to remember one temp sensor is just for the radiator fans and the other is for the ecu.

It might be worth researching which is which and if you can swap them to test if it's an issue.

It sounds like the cold start is working fine but it's switching from the cold idle to normal idle settings too early?

Check all your Earth points
Under the wiper motor, under the heater hoses , back of the inlet manifold
and the main earth on the back of the PPF (under the car near the diff)
Remove them all and give them a clean with emery paper and reinstall .

Earths on EFI systems are very important