Recently my Miata has begun overheating on the highway. In traffic or below 70mph the temp is steady. If I go faster than ~70mph it starts to creep up and only goes back down if I turn on the heat or slow down to 65.
From what I have read of other experiences with this the radiator is the culprit.
Has anyone had experiences with this? Keith?
Sounds like the radiator is plugged up.
Yep, I had the same issue when I bought mine a couple years ago. Went the the complete cooling system. A new radiator fixed the problem. I was going to go with an oversize radiator but was talked out of it. The argument being that the original size should be more than adequate and oversize would take me out of stock class autocrossing and that being the only non-stock part I would definately be outclassed. That and I was new to autocross at the time. So I put in a stock Koyo radiator and haven't had any issues since. Works just like it should.
Just put the top down and use the defroster. 
Since the heater brings the temp back down you're short on cooling capacity. A guy on a Nissan board had gotten a grocery bag stuck to the radiator and it had melted into the fins. New rad and all was well.
Personally I'd check the cap first, cheapest, simplest solutions first, and if you need a new rad it's a good idea anyways
Same thing happened to me. I fixed it by replacing the water pump that was leaking water at the time, and while I was down there, the timing belt. When that didn't fix the problem I still ended up replacing the radiator and all the hoses, some of which were pretty rank because they, and the car were all over 15 years old.
In other words, if its an old car. Replace everything that is worn out.
An automatic Miata radiator is a cheap upgrade.
I'd agree with it most likely being the radiator, but I'd also check if the water pump as been changed at the same time as the cambelt. They#re supposed to be changed together but some cheapskates try to save a few bucks by not doing that.
The belt and waterpump were changed around 120k (170k now). I figured it was the radiator but just wanted some reinforcement.
Stock Miata radiators have plastic end tanks. The usual failure mode is for the tank to crack, which prevents the cooling system from building pressure. Have the cap checked and the cooling system pressure checked, it's an easy job that will tell you quite a bit.
The automatic upgrade is a good one - they use a dual-core rad with approximately double the capacity. At least, that's the case with the NA models. I must admit I don't know if that's true with the NBs.
what if the cooling fan will not turn on when warm, but when the AC is turned on, both fans turn on?
What year?
sounds like a bad coolant temp sensor (plugged into the top of the thermostat housing)
skrzastek wrote: what if the cooling fan will not turn on when warm, but when the AC is turned on, both fans turn on?
If it's the cooling fan, it should be having cooling problems at low speed. Last time I took my radiator out, I forgot to connect the fan, and only noticed it as I pulled into the pits at the track (temp was fine at speed).
I've had my stock radiator crack.
I've also had a major cooling issue caused by a bad radiator cap. It couldn't hold enough pressure, so it blew and vented water prematurely.
Keith wrote: ...I must admit I don't know if that's true with the NBs.
You mean there's something that Keith doesn't know about Miatas? Oh, the disillusionment!
It's a 90. The coolant temp sensor has been replaced so that is not the problem. I will try a new cap and if that doesn't fix it I will go shopping for a radiator.
1990s have three temperature sensors. One on the thermostat neck that's just a switch to trigger the fan. Two on the back of the head, a single-wire one for the gauge cluster and a two-wire one for the ECU. To test the themoswitch for the fan, disconnect the wire and ground it. If the fan comes on, it's the switch. If the fan doesn't come on, it's something else.
I just did some work on a friends '03 Evo 8. It had similar issues to what you describe. I drained the radiator and did a through flush of the system, but the issue was still there. So i thought the next cheapest possible cause, the thermostat. Upon inspection the plunger that opens the coolant flow was bent slightly and was fouling on the housing making it run up to 3/4 the way up the coolant gauge. Installed a new thermostat (although its too small diameter and a new one will be here in the morning) and all is well. Also someone had filled the system with straight undiluted antfreeze, no a good idea!!! Def check out the thermostat!
Keith wrote: What year?
'90
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