I'm having an unusual issue with my recently bought 94 1.8 Mazda MX5 - can anyone help shed some light on what's going on?

Basically whenever I fill up the radiator and header tank to normal levels (ie rad to brim and header tank to between low and high) after a run or even 10-15mins of idling the header tan overflows through the overflow pipe. When the engine is left to cool, you can hear loud gurgling noises as the radiator sucks back in coolant as it cools and depressurises. But this leaves the header tank empty as effectively a header tank full of water has been lost through the overflow. The car then seems to settle at a stable level with a pretty much empty header tank and a full radiator. After a run the header tank will be almost to the full level whilst still hot, but then empty out after the back-pressure sucks the coolant back.

I don't think there are any radiator blockages as the car shows no signs of overheating - the temp guage works and stays at a stable left of centre position. The cooling fan only comes on occasionally. None of the coolant hoses show any signs of leakage and there are no puddles of coolant under the car on standing. Did an oil change this weekend and no signs of water in oil, so pretty certain no head gasket issues. I have been told by a mechanic that it looks like someones has fiddled with the car and the coolant hoses aren't OEM and should be replaced.

Just can't understand why the system dislikes running with a topped up header tank and always forces that out of the overflow? Any ideas on why the header tank overfills when the levels are at normal but no engine overheating problems?

Where does the coolant level settle wrt the to max and min marks? If it's between them, you're blowing excess coolant out of the overflow.

Where does the coolant level settle wrt the to max and min marks? If it's between them, you're blowing excess coolant out of the overflow.

The coolant level seems to stabilise with a full radiator and an empty expansion/header tank. Put any coolant in the tank and it blows out after a run.

Just checed today and the radiator itself took about half a litre to come up to the top, so there's something not right...

sounds exactly the same problem as in my octavia, it seems to happen after q quick run but only when i suddenly slow into say a 40mph limit, seems to be dumpig out of my overflow, i have no idea where to go now!!