Magento website - bug resolution

Urgent:

We have a Magento website that has been customised to allow customers to add accounts and shop for individual categories, depending on attributes chosen..

There are issues that have been reported that need to be resolved as soon as possible.

Summarised below:
1. The backend of the website is slow. Currently when uploading products the site times out.
1. Customers are not able to add categories and hence, can not shop
2. Customers who register and receive the 'thank you for registering' email are not able to log in to the website on next visit
3. Customers add products to their shopping cart, only to find it's empty when viewing it
4. Customers who spend over £150 are being charged a P&P cost even though the backend has been set to 'free for orders over £150)


This job is for a reliable developer and we will have further work for the successful applicant.