So I bought an '86 Mazda 323 LX hatchback with 198k km on it.

It has the 1.5L carburated engine, and I have a slight issue with it boiling over and pushing coolant out of the reservoir after I stop it. It has gotten a complete flush and new coolant with 50/50 ethylene glycol mix.

When I drive it, even when driving it hard, it does not go over the middle of the temp gauge. Even when idle, the fan kicks in and out and temperature is ok.

However, as soon as I stop it, even after just cruising for a while, I can hear boiling start to appear from the upper hose of the radiator or thermostat housing. It then fills the reservoir with coolant until it overflows. My only fix is to let the fan run after I've stopped the engine, then start it to let the cooled coolant flow through and stop the boiling.
If I just let it idle, the fan will start and stop as usual, but when I stop it, it still boils.

What could be causing this? I've never experienced this on a car before. I suspect this car got a new headgasket in 2019 and there was a spare thermostat in the glove compartment, so it has had some overheating issues before. What's weird is that it isn't a problem until I stop it. I've tested the thermostat and it seems to be in working order.

Thanks,
Lars