pressure in header tank when engine is stone cold

[racing green rover]

Hi Iv had a problem with water loss and slight misfire when engine is cold whet I press the accelerator quick I was thinking inlet manifold gasket was leaking first but when I unscrewed the header tank cap this morning there was a hiss and the coolant was on MAX this time I'm wondering if the pressure has raised the level when cold the engine doesn't overheat it usually reads around 88 degrees when running car otherwise runs fine apart from above problems the oil and water are clean no sign of any mixing I can't smell any exhaust gasses in the header tank either
car in question is a 1.8
Thanks