pressure building in cooling system?

I have a 1980 datsun 280zx with an L26 motor out of a datsun 260z and cannot keep it cool. It builds so much pressure in the cooling system and it is blowing my heater lines and radiator hoses because of the pressure. I tried it with the thermostat in and out and it is the same both ways. sometimes when i try to pull the cap off after bleeding pressure off the radiator the cap is suctioned down. It has the right pressure cap on it also and is new. Thank you