why does my vauxhall astra 8v have a starting issue?

I have a 2003 8v astra estate 1.6 automatic, it sometime starts straight away then other times takes a wile.... but when it take time it also feels as if it is trying and pumps out!! and also feels like it is cutting out or going to die. when it starts it runs fantastic!! can anyone help as i am selling it and don't want to let it go with this issue??