Mazda MX-5 gets hot

Hello, My mother has just imported a MX-5 from Germany (we live in Spain). BJ 2001 so Facelift. This is a 1.6 liter with 110 hp. The problem is the car gets hot when it is kicked a little bit. If I drive 120 on the highway the temperature indicator remains exactly in the middle. But if I give a little more gas (140-50 KMh) the temperature starts to rise to maximum. Then I can either set the heating to maximum power (then it cools like I don’t have any loss of coolant, even a white “steam” from the exhaust… And the power is also right. Please ask for your tips. And if someone had to change their head gasket, ¿What did it cost? Thank you…