Webmaster for new site

We are looking for an independent web programmer/webmaster to work on our newly launched, subscription-based site. We need somebody who can fix specific coding problems on demand. All the jobs will involve improving the site either by adding new features which we request or fixing current ones.
Currently we need to fix the subscription page, optimize the inbuilt search on the site, and look into why there are sometimes problems with our confirmation emails when sent.
We do have website developers, however they do not specialise in fixing specific problems and to get them to act is a very long-winded process. We believe that the site was largely created in PHP so applicants should have a strong grasp of PHP and other web programming languages. Applicants will need to be able to analyse the site\'s workings for themselves so that they can then fix things.
Payment will be per problem and will vary depending on the scale of the specific project and time period. Above all we need punctuality and efficiency.
This will be something long term but a very much part-time position.