Coolant Problem

[pocusfocus]

omg the car is perfect but driving back home from the guy who i boght it off on the mway it lost power

then the temp went up i stopped opened the bonnet and the coolant tank was leaking from the top with load of pressure.

so i thought it may be air so i bled it for half hour, the guy did say he aint drove it in three weeks. after this it was normal until i went up to 80mph it did it again temp up and coolant pressurised.

the first day i bought it it messed up i cant belive it i dont know what it is

i checked engine no mayo soo cant be the head the heaters blow mid air not that hot but sometimes they do

is it water pump i dont know..

please anyone here who had same prob plz advise me thanks