How do I fix my hic-cupping Vauxhall Tigra (N reg)?

I have replaced the plugs/air filter/fuel filter and have had it on a diagnostic machine which didn't find any fault. When driving it starts to 'hic-cup' and will stop at a junction (not every time) it is slowly driving me mad. I've only had it for 5 weeks and feel like driving it over the cliff. Can somebody please help? Thanks