MattS: Have you got a test site? not on your host.

Try using your test site,set up a store and load your EP file and see if it works on this(as it would be independant to your live site) if so you would know if it is a hosting issue or not, or whether it is a setup issue.

The thing that seems strange is that you are unable to upload just one product, which to me seem to be a hosting issue as the mysql query is very small, and makes no difference if you have one product or 30,000 products in the database.

One question you don't answer is what is the reaction of your host to your problem ?