P reg corsa?

i have got aproblem with my 1.2 p reg corsa. when warmed up it does not pull smoothly not a definate misfire but a gentle hiccup.costant not a single action.
this does not happen when cold and more fuel is available. have changed fuel filter,dizzy cap,rotor arm,sparkplugs.not the leads yet.head gasket also changed due to failure.