The 500 error is likely the result of a bad database call/bad code in that package - it isn't triggered on all pages because it doesn't execute for all pages.

I'd recommend turning on "Debug mode" in the dashboard so maybe a better error message will be generated that allows you to see what is broken.

Start tracing the problem to specific pages and look at what part of the package is triggering when that page loads. Then narrow that down more and more until you find the specific function that's causing the problem.

It could also be a bad theme file that is causing a problem for a certain theme (if you're using a theme overrides in the dashboard).

Nice site design though! Like the horizontal navigation.

Adding additions to your log when package functions execute is a good way of tracking problem areas.

Here is a page with more details about logging
http://www.concrete5.org/documentation/developers/system/logging...