Coming back to the theme,

Some feedback from the administrator suggested it could be a memory limit or plugin related?

This link was also sent:

http://support.hostgator.com/articles/specialized-help/technical/wordpress/how-to-fix-the-internal-server-error-in-wordpress

The first point seems to suggest a php memory limit has been exceeded? Any ideas how to fix this? Is this theme related (as per your

Creating a new .htaccess file did not help. (no 1 on the link)

Please advise.