(This is not about a miata, but I included that NG because the general Mazda NG looks about DOA.)
One of my cars is a '90 Protege...great sturdy little machine and with over 182K miles, it runs great and is so clean under hood you could eat off the valve cover, but I digress....
Anyway, here's the deal, Nearly every time I start it up from a cold start, the temp needle begins to rise normally until it reaches about halfway between C and H and stops there, as it should, BUT then after about a minute or so, the needle begins to rise again, toward H. Almost always, the needle will rise fairly rapidly and get uncomforably close to the top end, then suddenly the needle drops back to halfway between C and H, where it should be. This happens like clockwork, at about exactly the same time after start every time. After it does its thing, I have no more problems whatsoever...until the next start-up from cold.
This car has done this off and on for years. About 10 years ago, it was doing it, so I replaced the thermostat, confident that this was the problem, but it still did the same thing with the new thermostat. It also has gone long periods without doing it. A couple of years ago, I replaced the lower radiatator hose, and it started doing it again...that time the needle actually went up to H and the car ran hot, with steam, the whole bit. After I parked the car and cut the engine. It behaved normally, as the car was not low on coolant, and it isn't now. The problem couldn't be related to dirty, old coolant either, because I flush and change as recommended. The only thing I can figure is that somehow there's trapped air in the system or there is a backwash of coolant near the top of the engine where the sensor is. Any ideas? Thanks.