Coolant boiling off in overflow

OK, I couldn't find anyone with exactly this problem, so I thought I'd post.

I have a 1994 Miata with 155k miles. I live in North Central Texas.

I noticed that my coolant overflow was showing up low or empty rather frequently, with none of the usual signs of coolant leakage, etc.

I then noticed that after a longer drive, particularly in warm weather, when I stopped and got out of the car, I could hear a gurgling noise under the hood. This proved to be coolant boiling in the overflow jug. I believe that coolant is boiling off from the jug, gradually lowering the system coolant level.

We did a deep rebuild of the engine last summer, after a blown head gasket incident (I had a pinhole leak in an obscure hose which drained the coolant out very slowly; pressure built, gasket went, etc.). During the rebuild, I also I replaced every major and minor cooling system hose, the thermostat, and the water pump. These are all stock Miata parts, no aftermarket. There is no sign of coolant leakage from any of these areas, nor is there evidence of coolant in the oil. Car runs perfectly.

The temp gauge sits at the perfect -shy of 12- position.

I tried replacing the radiator cap (which was well worn) with one from AutoZone - no change in the boiling.

Here's what hasn't been replaced:

1) Radiator (which, admittedly, is aging and looking a bit ratty around the edges). 2) Coolant temperature sensor (under the ignition coil, because it's a pain in the arse) 3) Coolant switch (the factory manual shows one on the thermostat housing, but my model doesn't have one, so I'm thinking that was from an earlier model year and I don't have one, unless it's hidden somewhere else). 4) Heater core

As an addendum, I did get some slight heating in the hottest summer days (up to 1 or 2 oclock on the gauge), but only when A/C was on. I discovered that my passenger side cooling fan had failed. I pulled this out, and haven't yet replaced it, since I don't need A/C right now. I was waiting on a new radiator to do both together. To the best of my determination, this fan -only- runs when the A/C is actually on, so it shouldn't be impacting my current coolant loss. At least, I could only get it to run by turning on the A/C, no matter how hot the car was.

Any ideas as to the culprit in the coolant overflow boil-off?