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.

stu.
January 2010